Your question is rather incomplete. It is not clear if you’re writing in English or in another language.
There is no notion of “document language” (or principal document language for multi-lingual documents) in ODF. Writer uses the UI language for the caption categories as explained in bug tdf#151020.
However you always have the capability to override the category in the Insert
>Caption
dialog. Just type whatever you want. But be consistent! Use the same custom caption for similar objects, otherwise you’ll end up with two sequences and two lists.
Captions are not “managed objects”. I mean that once you have created them, they are just ordinary paragraphs with a field inside. Consequently, there is no way to modify them from a central point. You must change them manually one after the other.
If you’re “fluent” with Writer fields, you can also create your captions manually, which gives you much more versatility on the caption structure. All that is required is inserting a number range variable field in the paragraph and wording of the paragraph can be made in any language.