How about a toolbar icon where a small HTML 5 page opens in which free text, e.g. German text, can be entered in a text box and then translated into language X, e.g. English.
Later perhaps with more features, something like Speech-2-Text support.
What do you think?
One question is this translation technology allowed to be used freely?
If so, then you could also provide an API for translating, a can but not a should or a must 😉