When I copy text from one document to another document the language setting seem to change. Why?

Details:
I am working with one document created by me, and documents created by others. All documents are written in Danish.

I want to copy text from my document to the other documents, keeping the format settings (color, font, section info, etc.) including the language setting.
But apparently the language setting are changed. My Danish text are changed to other languages after being inserted into the other documents. The inserted text appear to be written in English, German, Albanian, ...

What might be the problem? How do I solve it, without having to change language setting after each insert?