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On Google Search: Options -> Search settings -> Languages

There you can change the language(s) of your search result. I think you can mark up to 10 languages.

Except it still gives me mostly bad German results for technical queries if I don't completely remove German there.

Google provides those settings but it seems to randomly ignore them or at least I can't discern a pattern.

It's not randomly. In work I use Iceweasel for private gmail (and Chrome for company gmail), where I have set Results per page to 100 and chose to Never show Instant results long time ago. It simply does not work!

I go to google.com, start typing and Google starts showing me some instant results and hitting Enter gives me only 10 results per page. (The same settings are respected for non-private gmail in Chrome, though.)

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