Okay, after trying to customize my keyboard layout to work around this, I came to the conclusions that you guys borked this internationalization thing completely and never actually tried it on something besides a en-US layout.
How do I know this?
Easy. I went to the tuner and tried it out first with the squared bracket character. First of all, by the way, the tutor is misleading for this one because he doesn't indicate that you'd have to press the NUM-key. But even if I do that, I get the ü character.
That's where it hit me: You're obviously not sending the ASCII or UTF-8 code for a character. But you're sending the key code for the key where the [ character resides on a en-US keyboard. On a de-DE keyboard, however, the ü character resides on this key.
Now, there's an obvious solution: Switch to the en-US keyboard layout, enter the key combination for where the [ char resides on a de-DE layout, getting some completely different character in the process, then switch back to de-DE and bam! I get an [ on a de-DE layout.
There's only one problem: de-DE keyboards have the [ on a key which I reach via Alt Gr 8.
And this key combination doesn't exist on en-US. So, no [ (and ] { } \
) on a German layout for me. Or I can simply try to cope without Umlauts. Great.