The language operations in the modeler allow you to copy from language A to B overwriting all existing values that are both in A and B.
A “copy missing values” would be very helpful e.g. when you decided to switch from default language when modeling, when you have imported part of your module from another module or when you have different developers working in different languages on the same project at the same time.
Indeed a merge option would make multilanguage development a lot easier! +1 from me.