Language is one of the metadata tags you can apply to books in calibre. If you change that tag, it will be reflected when you re-send the book to your device.
If you need more granular control over the language, calibre includes an
ebook editor, which you can use to style specific tags with language attributes.
I am pretty sure the Kindle can recognize the granularity, at least with the dictionary, so there should be no problem here too.
