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:
Every time a character gains a level, he or she can retrain. This involves changing one feat, power, or trained skill. Only one change is allowed per level. If the player book says to replace one of the character’s powers with a different power of a higher level, replacing the power doesn’t count as retraining. The character can still retrain a feat, some other power, or a trained skill.
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.