The story I'm writing is in English, but it's set in a constructed world with a range of different languages. The general conceit is that anything the viewpoint character (of which there's more than one, with more than one language) understands is portrayed in English.
The trouble with this is that many of my characters speak in a fairly informal way (in a "less formal register" of their own languages). Intuitively, I've represented this through the use of somewhat informal English. So, these characters say things like 'y'know?' and 'alright', and 'yeah'.
These phrases are also used in different ways by different characters, helping (I hope) to give a clearer picture of the sort of people these characters are.
My concern is that these specific phrases are unlikely to be directly translatable in the fictional language, and so by using them in the "translated" final text, I'm misrepresenting what my characters "actually" said.
I also find it a little jarring when English idioms turn up in translated versions of non-English novels. I find myself diverted into trying to guess what the original phrase was, and my immersion in the story is broken. I'm concerned that my fictional translations will have a similar effect on the reader.
I'm torn, then, between using phrases which break realism (but add nuance to the characters and how they relate to their culture) and not using these phrases (and losing those nuances).
Another possibility I've considered is to do direct translations of the phrases they would more likely have used, but since these will only be meaningful when the reader is already familiar with them, I'm concerned that I'll need to do a lot of setting up (and the reader will have to do a lot of adjusting) before this can even begin to express the subtleties that the English equivalents can.
Are these my only three choices? If so, are they really as problematic as I think they are, or am I overthinking this? If not, what should I do instead?