How do i change the language in the spell checker from German to English US?
You can't set a default spelling dictionary in Scribus (as far as I know).
Spell-checking is done at a frame level rather than a document level.
What you need to do is make sure you have the dictionary installed for the language you need - sounds obvious but if you don't you'll get very strange results - then set the same language in the Character/Paragraph Style you're using.
If you don't set the correct language in the style then Scribus can't spell-check it properly. The same is true if you don't use styles; Scribus will use the default language of the default style which is probably your Scribus language setting from "Preferences / General / Language".
This might seem awkward for a document in only one language but if you use styles correctly it's not too much extra work.
If you really don't like using styles and need to create single-language documents in different languages you could try changing the "Preferences / General / Language" setting but that would change the UI text too and would also limit the dictionaries you can use. It's a work-around but maybe not a very good one depending on what you want.
See this post https://bbs.archlinux.org/viewtopic.php?id=178135 for a bit of extra info.