The BBC reported recently that Shanghai is trying to get rid of “chilglish” (English poorly translated from Chinese). If you want to see some examples see their slide show.
Sometimes a bad translation is cute, “you are hear,” or “beware of the bee.” Other times they are funny like “don’t fall down,” instead of the more standard “watch your step.” Sometimes they leave folks scratching their heads like “Forbid hatchet man machine.”
Of course english speakers make mistakes translating too. The point here isn’t to beat up non-native speakers. It’s about how important it is to do it right.
Signs are only the most obvious places (T-shirts being the second most obvious). But there are other places where translation is important. For me, the most important place is literature.
I couldn’t get through Crime and Punishment until a friend pointed me towards the translation by Richard Pevear, and Larissa Volokhonsky. Another example is the new translation of the Little Prince that may (or may not) be worse.
Some people think that machines will save us, and that it is possible. I doubt that a machine will ever be a reliable source of translation for anything more complex than a set of instructions— at least not in this lifetime. Even with decent machine translation it won’t save people from doing stupid stuff like this:

Until we all get super computers from the future, or magic powers that let us understand the nuances of every written language translators are essential to sharing culture across borders.