The canonical place for language to come from is as an applied attribute, ie: each bit of text has been formatted with a particular language. However, not all applications allow for such things, or provide that information when saving a file, copy-pasting, etc. In absence of specific language information, Nisus Writer will assume the text is in your dominant OSX system language.
So your solutions are to do one of the following:
1. Adjust your OSX system language order.
2. Paste content as text only. That way the pasted content will assume all formatting of the destination document, including language.
For more on this, you might see this FAQ:
How can I prevent the US English language from appearing?