I figured I'd ask this question sooner rather than later. The characters in my vampire WIP come from a variety of linguistic backgrounds, the most important being 13th-century Scottish Gaelic and 14th-century Middle English. The hero of the story speaks the latter, and a friend he will meet in a chapter or so speaks the former. The hero's Middle English is written out as plain modern English due to Translation Convention, but I'm not sure what to do about the Gaelic.
I'm not a linguist, so having long passages of dialogue written in Gaelic would be nigh impossible for me without a reliable translator's help (and probably a glossary for my readers). I use a smattering of Gaelic terms throughout the story (as well as my other books involving Fae), but they're not nearly so obtrusive as such dialogue. I've seen it in published works before, and having to go to the back of the book multiple times every few pages, or even multiple times per page in some instances, got a bit annoying. And something tells me that the trick I use for this issue when RPing (writing foreign language text like <this> and English [or the native language of the protagonists, if it isn't English] text like "this") wouldn't work in a published novel.
The late Brian Jacques used a different technique in his Castaways of the Flying Dutchman books; he simply wrote every single language out as English, probably because the main character is an omniglot. Terry Pratchett did this as well in Nation, between an English-speaking girl and an unspecified-foreign-language speaking boy (which got confusing when they weren't supposed to be able to understand one another despite both their dialogue being written in English).
Any suggestions on what I should do?