Have you tried searching for the hashtag #cymraeg? I occasionally post in (bad) Welsh and tag those posts as such, and have found others who do the same. Also, my Instagram is entirely in Welsh. I use it as a practice ground as it makes me think about what I’m trying to say and how I can say it using the Welsh I know.
Yup - I find it patchy - the last time I looked 6 of the first 10 results were in English, and 2 more were bilingual… and, of course, you don’t get hashtags showing in your timeline, which is why I’m thinking having a specifically re-tweeting account would have value (if we can get enough people tagging it, of course… ).