OK, we're unable to replicate the local mp3 file play issue so far, but I found a solution for the Russian language issue (at lease it appears to work for me)
Create your text messages as such: <lang xml:lang="ru-RU">Мой текст здесь</lang>
Probably the traditional "language-region" format, which for United States English would be en-US. Guessing the supported list for Hubitat's serivce, reported to use Amazon Polly, would be this: