Sometimes LibreOffice is just too LibreOffice... Where do you change the spell checking language for the entire document?
You select everything and then change it at the bottom? Nope. A new paragraph will not follow that.
File -> Properties... which opens up the Document Properties Dialog since apparently this is a property of the document? Nope.
Tools -> Options... -> Language Settings -> Languages -> Default Languages for Documents and then check the "For the current document only" checkbox in this section, which will make it so you don't change your global setting...
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(I found this out by reading the manual, by the way: https://help.libreoffice.org/6.2/en-US/text/shared/guide/language_select.html )
