Cache results of available languages

This function is about 8 times calles for every single page call, when caching this variable I was able to gain a small performance improvement from 20,512 µs to 708 µs. (profiled with xhprof)

Surely, this is no gigantic gain but if we would do that for every function out there...

Before:
![screen shot 2014-11-27 at 00 07 35](https://cloud.githubusercontent.com/assets/878997/5210254/7e443d8a-75c9-11e4-9087-7d071a082666.png)

After:
![screen shot 2014-11-27 at 00 09 21](https://cloud.githubusercontent.com/assets/878997/5210265/acb0c3b4-75c9-11e4-827a-7054eb75147e.png)