I saw the question Is cross-posting permitted and I am wondering:
What's the deal with multiple languages?
For example asking something on the the German TeXWelt (of course written in German) and on the English TeX StackExchange could make sense to reach a wider audience.
Yes, that's fine. The non-English Stack Overflow sites even have a Help Center article about this. For example, the Spanish one:
Should I translate questions from English to Spanish?
Of course!
If you find a useful question on one of the sites of the Stack Exchange network (for example in Stack Overflow in English), which has not already been published in Stack Overflow in Spanish and the question is valid as described in What topics can I ask here?, please translate the question and the answer(s) to Spanish and publish it on the site. Our manifesto:
Create a world in which, for any query to a search engine, the search results show the link to a detailed answer in Spanish.
Translations increase the knowledge available in Spanish and help thousands of colleagues understand and solve problems currently limited by language.
(100% Google Translate, I'm surprised by its accuracy.)
Note that this is about cross-posting within the network; what you describe is another, less often seen form. In either case, it's probably a good idea to link the questions to each other (and if you copy a question from someone else, you are even required to provide attribution), and if any answers are posted, ask the author to post them on the other site as well or do it yourself (again keeping in mind the attribution rules).
If its not a SE site - it probably doesn't matter - and TexWelt looks like it runs on a clone. There's lots of situations where someone finds an answer offsite - on fora or even chat, and closes the loop on their own question.
Within SE, if its in the appropriate languages (say english language SO and one of the localised SOs) - chances are the overlap is small, and there's potential value in the answer being translated so... it seems like it should be fine, especially if you follow up on the other site you post on.
Re-posting on texwelt.de will probably not increase the audience by much because many (most?) of the users there are also users of tex.se
In general x-posting without mentioning the other post is not well received on tex.se, the least you should do is to include links to each other in both questions to avoid that the users trying to help you wast their time with an already solved problem.
(this might be handled differently on other communities, this is just my experience from back in the old days when I have been a user of tex.se)
