Joan's trial was in France, and the transcriptions are primarily in French, although some documents are in Latin (still the "official," learned language). A friend directed me to the contemporary description of her clothing at the time of her arrest, and a colleague did the translation, after which I had the costume made by a husband & wife team who make excellent medieval and Renaissance clothing and armour. I highly recommend them for their excellent research, meticulous attention to detail and overall amazing quality. You can check out their web-site here:

http://www.historicenterprises.com/

In terms of French vs English language use in England, after the Norman Conquest the lingua franca of the English court was typically "Anglo-Norman" French, and there's no reason to believe that any English king from the 12th-early 14th centuries made much use of the "native" tongue (in fact it's well-established that Richard Coeur de Leon didn't speak a word of it, and why should he have, since he spent less than 6 months of his ten-year reign in England?). It was only in the mid to late 14th century that vernacular English started appearing with any frequency in literature (think Gower, Chaucer, etc.), and it wasn't until Henry IV (1399-1412) that official documents, proclamations, etc. were issued in English. It's no small irony that the amalgamated--albeit primarily Germanic--tongue of this oft-conquered island became arguably the most influential, if not most widely spoken, language in the world!

~D