I haven't used Word for a while so I don't remember exactly where things are but I think I can help.
Whenever you create a document it's based on a template.
The template gets its settings from the styles that are used when it's set up- headings, fonts, margins, page breaks etc.
So assuming the original template had German as the default language, then you can select text and change the spell check setting but it will default back to German.
Thee are two ways you can fix this. One is to select all the text and then go to the setting for 'character' (this is probably in the 'Format' menu from memory). There you will be able to change the default font, style, size and language.
The second method it to open a new document- just as you do anytime you want to create a document.
This will use a template in English assuming you've never seen this problem on anything you've produced yourself.
Now select all the old document (Cntrl+A) and copy and past it into the new document (Cntrl+C, Cntrl+V)
I assume those are right for Word still?
You may have a few formatting issues as your template will probably have different tabs, page breaks etc, but it should be in English.
It's worth spending a bit of time learning about templates if you create documents because it saves you so much time eventually.
You set up a template exactly how you like it then every document will have the same style and settings.
Sorry I don't have a copy of Word in front of me any more to give you more details, but I think you should be able to work it out from there.
Best
