Translation:I have to lose weight so I am on a diet.
Oh yeah, now I see it. Thanks.
For me the prob though is that "so" could be "I have to lose weight, so (that's the reason why) I am on a diet", or "I have to lose weight so I fit into this dress". So = "therefore", or "in order to". I would change the "so" in their translation for that reason to "therefore".
English speakers do not say "on THE diet", but "on a diet". When translating from one language to another, consider the meaning of the original (before you translate it) and then express it in correct and natural terms in the target language. Je suis au régime = I am on a diet.