In frontend development, it's pretty common to use German to stress test UIs, as it's a commonly used language with a lot of long words. It reveals all sorts of UI issues - text that doesn't wrap properly, buttons that are pushed off the screen, etc.

I don't know German, but I do like the idea of using compound nouns rather than inventing new words.

It's also common to test UIs with Arabic, to test handling of right-to-left languages. At work we have a "fake RTL" language that's English but rendered RTL. It's interesting because everything is flipped - for example, a left-hand sidebar is instead on the right side in RTL languages. Company logo is at top right instead of top left.