Hi, Bill Chan,
The add-on itself doesn't impose any limitations. However, Google's Translate service, which the add-on uses, does have limitations. Google doesn't document those limits. So, I can't tell you what they are exactly. But they're quite high, so most users never hit them. You can only reach those limits if you try to translate really big amounts of text, something like thousands of translations and tens of thousands of characters.
Honestly, I can't imagine a scenario where you'd need that much translations at a time.
Could you please share your use-case with me? Maybe I could think of a specific solution for your situation.