"Is Dewi Lingo ill today?"
Translation:Ydy Dewi Lingo'n dost heddiw?
It is really inconvenient when one is expected to know phrase like "yn sal," that one could not possibly have. Is this another regional variant?
Yep, "yn sal" is the northern variant, and I'd recommend learning it since until I looked at the course I had no clue that "dost" even existed.
Sâl, tost, gwael are all used for ill, poorly, etc. (Sâl with the to bach accent.)
I put "tost" in this one and was told that "dost" was another translation. Does this mean that the mutation isn't that important here, or is it a glitch?
The mutation is required. Without seeing the question, your exact answer and the exact response by Duo, it is hard to say if there was a system problem or not. Using tost here instead of the correctly mutated dost is something that Duo's algorithm might flag as a mistake or simply as a typo.
If you do come across a system bug, please report it o that Duo staff can investigate - https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug-