The English translation is unnatural - firstly, a future tense here would be normal and also 'nain' is not English - surely nan or gran or similar. And does the Welsh mean 'you will call in to see gran tomorrow'? Or does it mean literally you will go past gran's house? It's not clear.
We deleted this sentence weeks ago and so it's frustrating that it's still appearing.
The intention of the contributor was a sentence along the lines of 'You will call on your Nan/Gran tomorrow', but they've translated literally from South Wales English, where the Welsh words 'nain' and 'mamgu' are still used.
It would be better as :- 'Galwch chi ar eich nain/mamgu yfory' = You will call on your nan/gran tomorrow.
Although even that is rather an artificial sentence and the command form would be more natural 'Galwch ar eich nain/mamgu yfory!' = Call on your nan/gran tomorrow!
I got that "nain" was common enough not to warrant translating as "gran", though I did stumble on translating nain at all. It seemed to come from left field, as it were.