Hello
So if you use the Bulgarian keyboard, you get these characters:
явертъуиопюасдфгхйклшщзьцжбнмч
This isn’t really right for Bulgaria, because т should look like m, and д should look like g, and other characters should look like rotated or mirrored Latin characters. E.g., г should look like a backwards s.
Compare the Bulgaria Wikipedia page in Bulgarian: https://bg.m.wikipedia.org/wiki/%D0%91%D1%8A%D0%BB%D0%B3%D0%B0%D1%80%D0%B8%D1%8F
with the Bulgaria Wikipedia page in Russian: https://ru.m.wikipedia.org/wiki/%D0%91%D0%BE%D0%BB%D0%B3%D0%B0%D1%80%D0%B8%D1%8F
Notice that the letters are different.
Anyhow, the ios Bulgarian font is just Russian Cyrillic, and that seems like an unintended bug rather than an intentional stylistic choice, basically.
Our engineering teams need to investigate this issue, as resolution may involve changes to Apple's software. Please file a bug report, include a references to documentation about the differences you have explained that can be used to understand the problem, and post the FB number here once you do.
Bug Reporting: How and Why? has tips on creating your bug report.