If anything, what you're doing is fairly niche and you aren't going to be distributing this addon anyway, only using it for your own personal use. So you can probably get away with using cheap hacks.
I don't know why your current method you are attempting does not work, and I probably wouldn't be able to help with that specific one, but it does seem like AnnounceInterrupts has a Russian locale. What if you opened both the enUS and the ruRU locale files and replaced the relevant English localization strings with the Russian ones? Cheap, but it would force your English client to output Russian in /y for this addon
(though it would be annoying to turn off/revert whenever you needed to go back to yelling in English.)
As long as enUS client would be able to handle outputting Cyrillic in chat on its own it should do the trick, I'd imagine.
Has your raid group been successfully outputting DBM announcements in Russian? Because if the official download doesn't seem to have Russian locale it's always possible they have their own unofficial translation somewhere. It's also possible that I'm completely wrong and that the Russian locale exists somewhere in the code and I simply missed it.
Also I believe there may potentially be an issue where spell and enemy names may show up in English on your /y announcement messages, I'm not sure if overriding GetLocale could trick the client into giving the addon Russian-localized names. So for example you'd be yelling something like "Прервал Pangs of Guilt у Belac" in chat which may not be all that helpful if your teammates can only read Russian.