Is it possible to retrain languages? How does one do this in the 4e Character Builder?
RAW, No. Retraining in 4e is limited to skills, feats, and powers. From the compendium:
The character builder will not support it as such. You can just manually go in and change a language but I would only do that with DM approval ahead of time. To do that load your character, click 2: Select Race and then double click on your previous selected language to change it.