This is useful for members who use “Merge forums” on member settings:

This is related with other suggestion, and they can be mixed: Show the language of forum target when I create a topic or when I reply.
German language will be added to Numista soon (and Italian more later)! And it increases the possibility of accidentally posting new topics in a language other than the forum language (which is the interface language). For example, a German member with the interface in German may posting a topic in English (they want to post it in the English forum, but the topic goes to the German forum).
For minimise these errors can be create a select box (or radio buttons) with the language target (default value: the interface language) when a new topic is created.

Other advantage of this: we will be able to publish new topics in other languages without changing the language of our interface. Now, I have my interface in Spanish, but in order to publish this topic in the English forum I must change my interface to English.