Change of language in a document won't work

Whenever I change the language of a writer or calc document, or a part of one and save, as soon as I reopen it, the whole text is back to default, in the local language, and autospellcheck underlines everything. Changing the local language won’t do, as I write in a few different languages. And I want the spellcheck on by default. What am I doing wrong? The corresponding menu can’t be there purely for information?!

Maybe these posts can help:

Edit your question (don’t start a discussion at this stage) to mention your OS, LO version and save format (.doc(x) or .odt/.xls(x) or .ods).