Special characters (and MooTools) are ruining my life
Turns out that this is a really bad idea to try and do.
Programming languages are almost exclusively written in English (JavaScript being one of those) which means that even if you write your program in a different language, keywords like return, var, function are still going to be in English and you're still going to have to use them which would get confusing when using functions, constants etc. that have non-English names.
The best solution is to just avoid using non-latin characters in variable names all together.
Even thought it works in most modern browsers, it makes your code harder to write and more confusing.
Leave the coding to the English speakers.
Although I don't get the point of that (especially for german people generally comfortable with english and sharing the same germanic linguistic origin), the simplest workaround to avoid encoding issues is to replace those special characters with their latin counterparts:
fürJeder -> fuerJeder
verlänger -> verlaenger
Uncommon spelling but still correct