In my country there are no Islamic books in my mother tongue that let the people understand Islam. Is it lawful to take i.e. a book in German or other language, which I can properly understand, and translate it, saying "Bismillah" before starting to translate it?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If these books which you translate are authorized Islamic books which have been approved by the scholars and you are good at translation, meaning that you master the language you translate from and the target language, then there is no doubt that the translation is desirable. The translation can even be an obligation  if it is a means for doing the obligations that Allaah ordained on us.

However, if you are going to translate from unauthorized books or you do not master the translation, then, in such a case, you are not allowed to translate these books as this may lead to spreading innovations and deviations.

Concerning your statement about saying "In the Name of Allaah" before the translation, then this is desirable and permissible before each work a Muslim does.  However, this does not make the translation permissible if it lacks the conditions which we have mentioned earlier.

If the books which you are going to translate are about culture or science and so on, then it is permissible to translate them unless they contain a religious prohibition, or if translating them causes any deceit or fraud.

Allaah Knows best.