I only have one database / repository; no separation by language. I feel that my brain processes information in a single aggregated “Language”, which is not compartmentalised neatly into several “languages” that I have acquired so far in my life.
I don’t publish my notes (and don’t intend to in the future) so my use case might be different to yours.
I use whatever language that comes natural when writing notes. I mostly write in English but if the book I’m reading is in Japanese, the bibtex entry is Japanese and the title of the note too. The note content can be mixture of English and Japanese in this case.
When I’m using a double-byte language (in my case, mostly Japanese and can be Chinese), I tend to have some keywords in Latin alphabet (English) somewhere close to the top of the note or in the title for note search (via Org-roam or Riggrep) – this is because I don’t normally use Japanese/Chinese for text search.
File name convention is date and time, no title in the file name (for Org-roam notes): yyyy-mm-ddThhmmss.md/org. For other files in my system, I have yyyymmdd_title-in-english.ext.
Dates in filenames are important for me – sorting, and finding. I read somwhere that the brian has easier time recalling the sequence in time and associate it with something – along these lines. I have years of files named in this way, and it just works for me.