'that' is essential in the translation of this sentence, unless it is clearly implied.
A more natural English translation of this would be 'I believe that you are right' but that makes it harder to see the pattern in the Welsh sentence.
An even more contracted version in English could be 'I believe you are (you're) right'
As Hatakend asked earlier. Could this mean 'I believe that you're OK/allright? And if not, how would you say it?
You could use it for that, especially if you are emphasising that 'you' are OK, but that someone else isn't:
- Dw i'n meddwl taw ti sy'n iawn, a Ffred sy'n sâl/anghywir - I that it's you who's OK/right and Ffred that's ill/wrong.