Restrictive license of translations and documentation
The license of GeoGebra translations and documentations, at least in 3.0 RCs is Creative Commons Attribution+Noncommercial+NoDerivs. While the software itself is free software under GPL, this means that its strings and documentation are non-free (as with a free software or documentation anyone who receives the work should be allowed to distribute modifications of the work and to use or distribute itself or modified versions commercially). This makes a general GeoGebra distribution non-free.
Do you really intend not to allow modifications and commercial distribution?
Not allowing modifications of the documentation and language files means that if someone, under the terms of the GPL, creates a modified version of GeoGebra, they are not allowed to adopt the documentation to the modified work; what is more, they are not allowed to change strings.
I am not sure if the application and the language files form a combined work; I would say yes, but IANAL. GPL requires that if the covered work is combined with other works to form a larger program, the entire work must be licensed under GPL, so the other works must be under a GPL-compatible license, which does not pose further restrictions. If the language files and the application are considered to form such an entire work, nobody (except the copyright holders) has the right to redistribute the entire work as the licenses of the two parts are incompatible.
For releasing free documentation, especially of a GPLed software, the most suitable license is the GNU Free Documentation License, similar to the GNU GPL. Creative Commons Attribution and Attribution+ShareAlike are also free licenses but are incompatible with the GPL. I think language files are generally not licensed separately from the application.