Same here - I have filename.en.srt and filename.sv.srt for Swedish.
Perhaps something to ignore whatever is between the second last dot and the last dot?
Rename.This.notthis.srt
Would be great. Plex and others just show "Unknown" as subtitle language if you don't have the language code. Also, you can't have two languages. I myself prefer english, but the rest of my family prefers swedish so I usually have both languages in there.