Long story short; translators lie, mess up, don't know the language that well, or are for some other reason unreliable.
They can also smooth things over...
There's some old Doonesbury cartoons where someone (Duke) is an ambassador and his translator is speaking for him. You read the stuff Duke is saying, and the translator is saying: "He is bragging about the US Economy." "It seems that he is beginning to tell a joke. Yes, and it is a very offense joke. He has reached the punchline and will expect everyone to laugh" - and the entire audience laughs.
If language difficulties are important to your game then I can see translations like that happening.
I can see language being important when dealing with a close-nit community or if travelling, but today most places have someone who can speak a little English. As long as you cover the imperial tongues (the English, French, and Spanish languages spread because of their empires) you should usually be able to find some level of communication.
Richard