According to the following update notes mid-last year:
I’m an Iruxi and my “brother” is a halfling who was raised iruxi. We cannot figure out how to take off the halfling language and replace it with iruxi, but according to the above post, that should be possible. Help please.
With our current version of the character tools, you can make changes in the “Other Languages” field but cannot make changes to the default languages granted during character creation.
I expect this will change at some point in the future.