With other ALT-key I mean the right ALT-Key from the space bar, in Germany called ALT-Gr, I don´t know how it´s called on an QWERTY-Layout.
Pasting symbols from Windows´ symbol mapalsodoesn´t work, only the Num-Block workaround.

This is especially annoying when trying to write lua scripts because it's not possible to write characters like [ ] { }. I also had to use ALT+ASCII shortcuts.

So, Right-Alt-Combinations don´t work in the whole game?

Switching to english keyboard language under windows also doesn't work, but copying special symbols and pasting them with Ctrl-V fortunately works everywhere!

For all thos people, who can't insert special symbols:

There are two ways to do it, (a charmap is helpful).
(Windows users: charmap.exe)

1st possibility:
Get the ASCII-Code of the symbol you want to insert (charmap) and then hold down ALT and press the ASCII-Vaule on the numerical block (NumLock must be off), then release ALT.
Note that the ASCII-Code you enter has to be 3 digits, so you have to fill the front with zeroes.

For example, to get the @, which ASCII-Code is 64, you'll have to enter 064 when holding ALT or
for Tabulator, which ASCII-code is 9, you'll have to enter 009 when holding ALT.

2nd possibility:
Copy the symbol from somewhere else (charmpa again or another text containg the symbol), then perss Ctrl-v to insert it.

Note that the LUA-editor can only paste text by ctrl-v at the END of the script, not inbetween :(