Right, and the fact they have in those 3 quests the nature of the language, written and spoken, and then also throw in the blue mage book to further emphasis the language barrier suggests its more a recent thing to have a more unified language. Which the only representative you ever speak to of the Whalaqee tribe, so far is Ceadda, who is a young boy and speaks fluently. Which we don't know how long Martyn was with them for teaching them a language. So it creates a "chicken or egg" scenario. If its pre-Martyn then the teaching of the language would be easier for him because of the Turali language being very similar for talking. If it occurs during his stay it would start off hard to teach but become easier over time with the new Turali language. If its after, then it would have just been doing it entirely from scratch where a good deal of time is just trying to teach them a language.
The clothes thing is an easy thing to dismiss as its easier to get one culture to wear a shirt than it is to understand a whole other language.
Oh right the letter. Which is entirely in eorezan, possibly dictated as Wuk Lamat suggests. Which begs the same question of the chicken or the egg. Was the dictation done with the new turali language, or was it done with the old language and someone who acted as a translator. Which as one might guess we don't have an answer to.
EDIT: Maybe there is something in the japanese version of the language? Different words used for a similar yet different context? For both Wuk Lamat and the blue book.
EDIT 2: So yes the author of the blue book, in japanese, taught the Whalaqee the "common tongue". Its not an english-only thing. There are other quirks in the book, similar yet different to the english version, but that is a different tangent. So potentially there is something in Wuk Lamat's text in japanese.
EDIT 3: Confirmed in japanese, each tribe over there spoke a different tongue. Also the Turali language was created based on "commonly used words around the globe" which made it easier to communicate. Also still the same about the letter being written by someone else.