Is seems very common task nowadays to translate some texts (technical, fiction, and others) from English to other languages and vice versa. I looked at the list of current SE sites including Area 51, and I don't know which site to choose without a risk of being blamed in off-topic.
For example, the English language and usage site says it is not for "Translation and non-English languages" (FAQ). So it looks like I should create new proposal on Area 51, but looking at this site I see many suggested sites for specific languages, including, for example, German Language & Usage, which is intended for translation queries as well. I think all language sites should have the same policy, so, which one is correct? Either German (and other language) sites should not address requests for translations, or the English site should be extended to accept such requests.
Also I'm not sure which question can be considered as a translation request. For example, I have a very hard to understand English sentence. Is it normal to ask to explain it in English (English-to-English translation) on the "English language and usage"?
I thought about creating a proposal of a dedicated translation site on Area 51, something like Cucumis (which sucks and really needs to be transformed into something like an SE site). But the problem is that Area 51 does already contain a lot of proposals related to translation itself. For example, there is the Translation Tools site. I think it's not a good idea to have numerous sites for very similar matter. So it could be possibly more appropriate to extend the "Translation Tools" into more wide "Translation" topic, but I'm not sure if this is possible at all (technically and procedurally).
I think all language sites should have the same policy, so, which one is correct?
not necessarily. Each community has relatively broad freedoms to define what they deem on-topic and what they don't.