How can I change the UI language, and apply a different spell checking language to paragraphs?
There must be something wrong with this specific setup because on the other machines I’m able to change the UI language to all the installed on Windows, and spell checking language to all the installed dictionaries.
You can only use those languages for the UI of LibreOffice, for which you have installed the language packs. In the “Installed Apps” part of the Windows settings find your LibreOffice. There use Modify. The “LibreOffice Installationsassistent” opens. Choose “Modify” and then “Next”. The next dialog page has “Language of the user interface”. Open the tree and mark the languages which you want. Continue the installation.
Thank you! This works (UI languages).
UI and spellchecking languages are two different matters.
@Regina answered for UI.
Spell checking is entirely under your control and can differ over paragraphs or even words within a paragraph.
Styles have a Font tab where you can choose a language for this paragraph or character style. And the chosen language doesn’t need to be installed (of course, if it the dictionaries are not installed, spelling will not be checked but words won’t be flagged as faulty in your local language).
If your text is made of “homogeneous” paragraphs, i.e. they contain sentences in a single language, all you have to do is to define dedicated paragraph styles.
Alternatively, you can keep the same paragraph styles (and therefore the same layout) for paragraphs in different languages provided you apply a dedicated character style which may be reduced to the sole language specification. However, managing several character styles over text (assuming you want, in addition, also to emphasise words) is not really user-friendly in the present implementation and rather error-prone.
As a last resort, you can also direct format paragraphs or words with Format>Character, Font tab. But this is not recommended because direct formatting makes your life very difficult when you want to tune your document formatting as it shadows styling. The process becomes fully manual.
Thank you! This works for spell checking 
