I don't know if this is the correct location for this, but here goes...
The Becomer feat for changelings is supposed to add a language and +2 to a skill known by the race.
What the feat seems to be doing is actually adding the chosen skill as a trained skill instead of a +2 modifier.
Finally, there is no way to add the additional language.
It looks like the best fix for this feat is to make one for each race it can apply for. I've not looked at the actual code of the feat in HL, but it looks like a deceptively hard feat to fix. Here is a modified fix I made for another feat that will add a language to be picked by the character, but not limited as described in the feat. Also it adds 2 to diplomacy, which will at best give you a jumping off point.
<eval phase="Traits" priority="10000"><![CDATA[
hero.child[resLangSta].field[resMax].value += 1
#traitmodify[skDiplomac,trtFeat,2,""]
]]>
</eval>