One of the major limitations is it's lack of ability to translate into the informal form of address.
Everything in Google translate gets translated into the formal form. That's a problem if you're talking to someone who you've known for a while, or someone who you don't know very well but is of your own age group. In French, German, Spanish, Italian and most other languages there are formal and informal forms of address. I have been writing emails in French as of late, and most of the people I'm writing to are people that I already know well enough to address in the informal form. What to do?
I read a blog on the Google Translate help site that stated that if you put in "(informal)" in parentheses after the personal pronoun, that the program would translate into the informal formal dress. This is absolutely not the case. Google Translate merely translates the parenthetical word into "informel", in the case of French.
And then I started thinking: didn't we actually have an informal form of address in English? Yes we did! Old English uses "Thee" and "Thou" as the informal form of "you". I learned that rather late in life. I always thought that when we addressed God as "Thou" and "Thee", we were addressing Him with respect and deference to His greatness. In reality we were speaking God in the informal form; just like an old bar buddy! ( I really like that...). The language of Shakespeare has a similar connotation for English speakers. How often do Americans, when trying to sound very formal, feign an English accent, and use expressions like "thou art" and "thou hast" and "wouldst thou"?
The trick with Google Translate is to use "thou" and "thee" for personal pronouns. This is something that Google translate understands, strangely enough. Just be sure you don't also use words like "art" or "hast", "wouldst", etc.
If you go that far, Google Translate does some pretty strange things!