how can i change the spellcheck language in impress to the one i need?
By default its the language of your desktop environment/operating system. You can override it for new documents with tools->options->language settings->languages, see “Default languages for documents”. You can use format->character and change the language for selected text, and similarly for styles etc.
Thank you for your answer
For me, this works temporarily. If I close and open the file again it still uses the default language of my OS.
Using LibreOffice 3.5.4.2 in an Kubuntu OS.
No it does not work. This has bin a pain for the passed 10 years. If you have a document with mixed languages or a doc that you have translated you have to change every ord.