Ferstl,
Arduino is an IDE (the interface used to write and upload code on compatible devices like Arduino boards or RePhone in your case) only. The language is “the Arduino language is merely a set of C/C++ functions that can be called from your code”.
To get started go on SeeedStudio recipe page and take a look there … or better go see the Arduino official forum.
http://www.seeedstudio.com/recipe/
To go to default working RePhone you need to flash it back as I mention, the info you need is on the wiki (sorry but you should do at least some research)
RePhone is a great platform and his strength is not “use it as a phone” but the fact that you can program it to interact with external world …
Small project to have fun: attach the break-up board to it (no screen needed); attach a LED; write a program to light the LED when a predefined SMS is received …
Regards!