The Languages template does not seem to accept the value "lb". "lb" is the proper ISO 639-1 code for the Luxembourgish language and WikiTree has at least one native Luxembourgish speaker who would like to use it.
I don't see anything on the documentation that says the language selection is limited.
Could someone familiar with the template please have a look and see what is wrong, please? It would be much appreciated. Thank you!