I see for other languages that a fluency score is given, and it's mentioned in the Schools' PDF, too.
Where do I find this for Welsh?
Unfortunately the fluency score is not available for languages other than Spanish, French, German, Italian and Portuguese. These courses are Duolingo's "legacy" courses.
I hope this helps!
Like Alec has already mentioned, fluency scores are only available for the "Core Languages" (German, French, Italian, Portuguese, Spanish.) At the moment, they are not available for Welsh. However, even in these languages the fluency score is known to be inaccurate and untruthful. Why not go by how well you think you can speak Welsh, instead of what a website thinks? Along with many other Duoligo users, I ignore the fluency score. I would recommend that you do too!