The letters relating to the war are printed in the original French in Pierre Chaplais's The War of Saint-Sardos (1323-1325): Gascon Correspondence and Diplomatic Documents; all translations here are mine. I love the written French of the Gascons, which to me looks kind of Portuguese: for example, they wrote senhor instead of seigneur (lord) and often wrote names with a final 'a', for example, Segrave was Segrava, Wateville was Watavila, Bayonne was Bayona, Portsmouth was Portemua, Dublin ('Dyvelin' in English records of the time) was Dovelina, and Edward II was addressed as rey Danglatora (king of England). I love this, as it gives me some insight into what Piers Gaveston might have sounded like. :-) One letter sent to Despenser came from Piers Bernard, captain of Edward II's ship La Seint Edward, whose French almost looks more like Spanish or Portuguese: nostre seinhor lo rey ho lo podetz destorbar e Diu vos don grassi de qe bon cosseil lo detz; at mon semlant combin qe cosseilhedz; ab le graci de Diu avem sopesson si ed fei lo mariadge sobredist de guadeinhar brevmens lo reisme de Navarre; a sire Hues lo Despeicer le geuen sien dadas.
An interesting light is thrown onto standards of literacy among the fourteenth-century nobility by Hugh Despenser's statement in letters to Arnaud Caillau and Despenser's kinsman Sir Ralph Basset that he had read their previous letters out loud to the king, "point by point." (This does not mean that Edward II himself could not read, only that he didn't have to when he had men to do it for him.) Despenser's surviving letters are all drafts, which were seized from the Tower of London following his execution in November 1326. These are fascinating, as the crossings out and additions give an insight into what Despenser was thinking as he dictated the letters. In one, Despenser informed Ralph Basset that Edward II would go to Gascony with "a great and noble array" and, interestingly, with Robert Bruce, if the "business" between them - Edward opened negotiations with Bruce in the summer of 1324, presumably to avoid Bruce allying with France against him - went well. The draft of this letter reveals that Despenser at first named Bruce as 'king of Scotland' (sire R. de Bruys le roi Descoce), but then must have remembered that, oops, no, we don't acknowledge Bruce as king, and ordered his scribe to cross the last three words out.
Other letters show Despenser's supreme confidence in his position as king's favourite, his certainty of his hold over Edward and his boundless arrogance, which I don't mean as an insult but as a simple statement of fact. On too many occasions to count, Despenser wrote "It seems to the king and to us that..." or "The king and we think that..." or some similar phrasing. (Letters of the era often used 'we' rather than 'I'; that's not Despenser using the royal plural.) A letter sent from Sir John Felton to Despenser in November 1324 ends "And, sire, please send me the wishes of my lord [Edward II] and your own." Sir John Sturmy, formerly steward of Edward II's chamber and admiral of the eastern fleet during the War of Saint-Sardos, ended a letter to Despenser in September 1324 with "My very dear lord, may it please your high lordship to certify me regularly of that which it pleases my lord the king and you that I should do, and I will do it with all my power." Even Despenser himself seems belatedly to have realised that he might have been overdoing it: an April 1325 letter to Sir Robert Wateville, who married Despenser's niece Margaret Hastings a year later, originally stated "We have well understood that which you have sent us by your letters, which we have shown to our lord the king and to his council as you requested us. And it seems to our said lord and to us that you have acted wisely and advisedly..." but Despenser subsequently crossed out the words 'and to us' (et a nous). Referring to an 'excuse' which Wateville had been forced to make for some misdemeanour - Edward II ordered Wateville's arrest on 15 April 1325 - Despenser crossed out the sentence "which excuse our said lord and we hold as truthful" (quele excusacion nostre dit seigneur et nous tenoms pur verroie), and wrote instead that Edward "is much softened towards you and put out of his great melancholy."
Hugh Despenser's letters also reveal surprising flashes of humour. He wrote to Edward II's half-brother the earl of Kent, Edward's commander in Gascony and in desperate need of cash for the campaign, to explain why the ships carrying money to him had arrived late: "And truly, Sire, there is no other reason that the ships did not arrive on time with you except that a strong wind was against them, which we cannot turn by our own command." This is the origin of the belief that Despenser said "Even I cannot control the wind" which, to be fair, isn't really what he wrote, and clearly he meant it humorously - although it does also demonstrate his arrogance, with its implication that Despenser controlled everything except the weather. In the same letter, Despenser told Kent that he and Edward II were "as anguished and worried as we possibly could be" at the late departure of the ships, which sounds a tad sarcastic.
Arnaud Caillau told Despenser that Alexander Bicknor, archbishop of Dublin, was deeply unpopular in Gascony and that the people wished he had stayed in Ireland (which in Caillau's Gascon-flavoured French came out as vostra gent de Guasconha ne vousissent ja qe larchevesque de Dovelina fust venu au pais anceis voudreint qe fust oras en Irlanda). Edward II sent the Dominican friar Thomas Dunheved to Pope John XXII in 1325 to complain about Bicknor and his conduct in Gascony; two chroniclers got the wrong end of the stick - actually, they weren't even in the right wood - and thought that Edward was trying to procure a divorce from his queen. Some of the Englishmen in Gascony for the first time were not too impressed with what they saw there: one told Hugh Despenser in December 1324 that "in this country, one will find nothing much except wine" (en cest pays homme ne trovera gueres fors que vyn). Sir Thomas Gray, author of the later chronicle the Scalacronica, made the interesting statement that Gascony was the "country and nation which he [Edward II] loved most" (terre et nacioun qil plus amoit). Edward in fact never set foot in his duchy, but as it was the homeland of Piers Gaveston, maybe Gray was correct. One of Despenser's Gascon correspondents was the sergeant-at-arms Isard or Isarn de Lana Plana, he of the cool name who was custodian of the castle of Sempuy, who endorsed a letter to Despenser in November 1324 with "To his very noble and powerful lord, to my lord Hugh le Despenser the son [a moseinhor Hues le Despensser le fuiz], by his humble sergeant and servant Isarn de Lana Plana." (Lanneplaà in modern spelling, about thirty miles from Gabaston, where Piers Gaveston's family originally came from.)
Ralph Basset came up with an amusingly impractical solution for defeating France: he advised Hugh Despenser to "have the treasury of our lord the king searched, to see if you might not find an old remembrance touching Castile, because I have heard from some old people [ascunes auncienes gentz] that the king of Castile often claimed homage as far as the River Dordogne, and several people remember that he should have the right." Presumably Basset was hoping that, seventy years after Alfonso X of Castile incited a rebellion in Gascony with a view to taking over the duchy but subsequently agreed to the marriage of his sister Leonor to the future Edward I and renounced his claims, the regents of Castile might decide to enter the war on the English side and fight France for a share of Gascony. Despenser didn't even bother to respond to this suggestion in his next letter to Basset. Castile did, however, consent to send men to aid Edward II against France: Edward's cousin Juan el Tuerto ('the one-eyed'), lord of Biscay, told him in early 1325 that he was willing to raise 1000 knights and 10,000 footmen and squires for a year, or longer if Edward required. Edward declared that he "recognised the abundance of Sir John’s grace."  Hugh Despenser, desperate for Edward not to leave England to lead an army into Gascony in the belief that his life would be in danger in the king's absence, was hoping that Castilian and Aragonese soldiers would fight against France on England's behalf. In 1324/25, Edward II betrothed his elder daughter Eleanor to Alfonso XI of Castile, his elder son Edward to Alfonso's sister Leonor, and his younger daughter Joan to the future Pedro IV of Aragon (none of the betrothals came to anything).
Even when the men in Gascony wrote to Edward II directly, they often wrote to Hugh Despenser as well and repeated to him what they had written to the king: Arnaud Caillau began letters to Despenser in March and in April 1325 with the words "Very dear lord. I make known to you that I have written to our lord the king in this manner: 'Very dear and very dread lord...'" and re-wrote his entire letter to Edward. Raymond Durand, seneschal of Les Landes and another correspondent whom Despenser addressed as "our dear friend," did the same thing in January 1325: "My lord. I have written to our lord the king in this form: 'Very dear lord, I have written to you three times...'"
One of the most amusing aspects of the Saint-Sardos letters is the sycophantic way in which Hugh Despenser's correspondents addressed the powerful royal favourite. Even Despenser's social superiors the earls of Kent and Surrey were not immune from the urge to flatter him. Surrey only ever sent Despenser one letter, in May 1325 (and sent none to Edward II), but in it, he carefully addressed him as 'very dear cousin', trescher cousin, five times in five sentences. (Surrey and Despenser were second cousins once removed by common descent from Maud Marshal, countess of Norfolk and Surrey, and married to first cousins Eleanor de Clare and Jeanne de Bar besides.) Edward II's half-brother the earl of Kent addressed Despenser as 'very dear nephew' (trescher nevou) or 'beloved nephew' (tresame nevou) more often than would seem strictly necessary - seven times in four sentences in one letter of May 1325, in which he also declared that he had heard news of Despenser's "good estate, for which we devoutly thank God." (Kent was the youngest son of Edward I and Despenser's wife Eleanor de Clare Edward's eldest granddaughter, so Despenser was indeed Kent's nephew by marriage, although about a dozen years older than he was. Kent was one of the men who condemned Despenser to a gruesome execution eighteen months after writing that letter.) Despenser, for his part, began letters to Kent with "my very dear lord" and addressed him throughout as ''Sire', but never called him 'uncle'.
The Gascon lord Simon de Montbreton - who had arrived in England by May 1326, when Edward II granted him permission to marry the widow of the recently-deceased William de Braose, formerly lord of the Gower peninsula, and Hugh Despenser paid him twenty pounds for acting as his deputy custodian of Bristol Castle  - ended a letter to "the very noble and puissant lord, my lord Hugh le Despenser" in May 1325 with the line "Make known to me always your commands, which I am ready to fulfil with all my power." Montbreton wrote to "the very noble, very honourable and very puissant lord" Edward II in almost exactly the same way: "Make known to me, if it please you, your good commands, which I am ready to fulfil with all my loyal power." Arnaud Caillau had arrived in England by early 1326, and Edward gave him the Essex manor of Thaxted; Caillau's closeness to the king and Hugh Despenser is demonstrated by a message sent to Edward by King Jaime II of Aragon in 1326, which Jaime's messenger was instructed to share with four men only: Edward, Hugh Despenser, Ralph Basset and Caillau. Caillau's letter to Edward II informing him of this names Despenser only as 'Sir Hugh' (mossire Hues); as in Edward's chamber journal, there was no need to specify which Hugh was meant. Arnaud Caillau, for all his loyalty to Edward II, was definitely a Bad Lad: his rule as seneschal of Saintonge resulted in a very long list of complaints against him in 1317, he forced a French official of Philippe IV to jump out of a window into the street, so that he broke his limbs, in 1312, and in 1303 organised a revolt in Bordeaux and installed himself as mayor. 
Here are some examples of the opening sentences in letters sent to Hugh Despenser:
- To the very noble and wise man, his very dear and very honourable lord
- To the very puissant, very noble, very honourable and wise lord, if it please him
- My very dear and very dread lord
- To the high and noble man, his very dear and very honourable lord, Sir Hugh le Despenser, lord of Glamorgan, his bachelor, if it please him, Robert de Leyburn, with all the honours and reverences he can give. My very honourable lord, I beg your noble lordship, if it please you, to give credence to...
- To the honourable and wise man and his very dear lord and cousin, if it please him, his John, lord of Segrave, greetings, honours and as much very dear affection as he can give
- To the noble, wise, and discreet and powerful baron, Sir Hugh le Despenser
- To his very honourable lord, honours and reverences. Very dear lord, I thank you as much as I can for the amiable letters which you sent me and also, sire, for saying that you are glad when you have good news of me. And know, sire, that I am putting all my efforts and my work into serving my lord [Edward II] and you as well and as loyally as I may
- To his very dear lord, with all that he can of honours, reverences and very dear affection
Edward II lost the support of several of his Gascon vassals who took Charles IV's side, including the counts of Foix and Armagnac and Amanieu, lord of Albret - though Albret, who loathed his fellow Gascons Piers Gaveston and Piers' brother Arnaud-Guillaume de Marsan and resented the favouritism shown them by Edward, had been disaffected since at least 1309. Amanieu was finally reconciled to Edward in June 1326. On the other hand, Albret's son Bérard (died 1346) was one of Edward II's staunchest supporters during the War of Saint-Sardos, and it was said of him in 1325 that he was "more enthusiastic than anyone else in these parts about the service of the king [Edward] our master" and that "he has drawn more French allies to our side than any other man." His father disinherited Bérard in July 1324 when he refused to serve against Edward. Edward knighted him as a banneret in July 1326. 
I'll end this post with a very nice letter sent by Bérard d'Albret to Edward II in June 1325: "To his very dear, dread lord, your humble subject recommends himself to your very high lordship [la vostra treshauta senhoria]. Very dear dread sire, your humble subject signifies to your very high lordship that I have received your letters stating that I should come to you, which thing, very dear dread lord, is the greatest joy that I will ever have in my life, that is, to see you. And, very dear dread lord, as quickly as I can, I will set off to come to you. Very dear dread lord, may God keep your soul and your heart."
1) Calendar of Close Rolls 1327-1330, p. 9.
2) Close Rolls 1323-1327, pp. 344-345, 350-351.
3) Calendar of Patent Rolls 1324-1327, p. 267; E. B. Fryde, 'The Deposits of Hugh Despenser the Younger with Italian Bankers', Economic History Review, 2nd series, 3 (1951), p. 362.
4) Foedera, II, i, pp. 351-352; Malcolm Vale, The Origins of the Hundred Years War: The Angevin Legacy 1250-1340, pp. 168, 221; Pierre Chaplais, English Diplomatic Practice in the Middle Ages, pp. 210, 226.
5) Jonathan Sumption, The Hundred Years War: Trial By Battle, p. 201; Vale, Origins of the Hundred Years War, p. 240.