A great belt of this nature actually lowers the wearer's Charisma score by 1, but only with respect to non-dwarves and their ilk. 

The girdle causes the wearer to gain 1 point of Charisma with respect to halflings of the stout sort and with respect to all gnomes as well. Dwarves regard the wearer as if he or she had Charisma 2 points higher than before. 

The dweomer of the girdle also enables the wearer to understand, speak, and read dwarvish language.

He or she is also empowered to otherwise gain the racial benefits of dwarvenkind, i.e. +1 Constitution, saving throw bonuses based on total Constitution, 6" infravision, and detection/determination of approximate depth underground [...]. 

All apply only as long as the individual actually wears the girdle. 

Benefits such as additional languages and armor class against giant-type opponents never apply.