Translation:I have found out who is responsible.
I'm always a little amused when a Welsh verb+adverb construction is the same as in English, as here: ffeindio allan "find out".
I've also heard troi lan "turn up", for example.
"Ffeindio allan" is a direct borrowing from the English. "Dod o hyd i" or "Darganfod" would be a couple of alternatives.
Quite a few expressions like this are widely used in casual speech, even though there are usually other, neater ways of saying them.
darganfod is perhaps better for 'finding out' or 'discovering', and it is accepted here.
troi lan might be used for 'turning up' the volume, etc. Otherwise perhaps cyrraedd (reaching, arriving, etc) or ymddangos (appearing).
Going the other way, quite a few Welsh constructions play a part in Welsh dialects of English. Welsh expressions will sometimes pop up in otherwise English conversations, too, such as ych-a-fi, chwarae teg, shwmae, etc.