Include the slug transliteration plugin into the core of GS

[mvlcek]

I'd suggest to rather use the 3-parameter function strtr(string, from, to), then you just have to include two strings TRANSLIT_FROM and TRANSLIT_TO in the language files and don't need to define an extra array.
This makes customization much easier and uses the standard i18n features of GetSimple:

Code:

...
'TRANSLIT_FROM' => 'äöüÄÖÜ',
'TRANSLIT_TO' => 'aouAOU',
...