How do I change the default language? I’m using Calc and it’s stuck in Italian (I think I set Italian as the language when I was writing an essay a while ago) and the currency is stuck in Euro. Even if I go in Format>Cells>Numbers or Font and change the language, it will switch back instantly. I even downloaded the new version of LibreOffice and I’m still having the same problem. Every time I click on a cell and press the currency button it goes to Euro. Help 
Have you tried looking in Tools |Options | Language Settings | Language ? (If you use a Mac that is probably LibreOffice | Preferences | Language Settings | Language)
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I’m a fool and I missed the + sign on Languages. Thank you!
that’s just the placebo, the moment you select a paragraph, and select the default language it reverts to the UI or the one you installed
No it does not work. I would like to have all the programms, writer, calc, etc in german language
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You need a language pack (name varies among OSes and installation method, may be “local language package” or an option during installation).
Then Tools>Options, Language Settings>Languages offers several independant settings: User interface for the UI, locale setting for various features and Default Languages for Documents with Western, Asian and Complex text layout choices.
Check all options are correctly set.