"I saw you."
Translation:Mi wnes i weld chi.
Gwelais should be accepted, but the welais version is also accepted because the sentence traditionally would have been "mi/fe welais" (NOTE these are verbal particles and have nothing to do with the pronounes mi and fe), but since the "mi/fe" particle is often dropped in the south you can either say "gwelais" or keep the traditional mutation "welais".
Sorry I didn't explain that well.
Your sentence Gwelais i ti is 100% correct and is the standard translation in South Wales and taught on adult courses.
In North Wales they tend to use the verb 'to do' to make the past tense. 'Mi wnes i weld = I did see/I saw.
There are various places to put the object 'ti' in this more complex sentence, one common one is 'mi wnes i dy weld di :- 'dy.........di' has more emphasis because of the two parts of the pronoun. I saw YOU.