According to https://commons.wikimedia.org/wiki/File:Cyrillic_alternates.svg, some cyrillic letters have particular Bulgarian glyphs. According to other sources, some Bulgarian lower-case letters have ascenders not found in other variants of the cyrillic script.
Is there some SIL font, or some tunable feature setting (preferably for Doulos SIL), to write Bulgarian text with the proper glyphs? If not so, are there plans to add support for Bulgarian to SIL fonts?
Thanks for your interest in our fonts! We do not currently support Bulgarian versions of letters in Doulos SIL or our other Cyrillic-script fonts. It’s been on our wish list for a long time but we’ve always had higher priorities, so I can’t guarantee when we might add them.
Under the terms of the Open Font License (OFL), you are welcome to make these changes and release the font under a different name. As long as you do not use the names “Doulos” or “SIL”, you are allowed to make any changes you like to the source files. If you’re willing to do a set of characters for all our fonts then we’d welcome your contribution. Contact me if you’d like to find out more.