Similarly, ja_JP, ne_NP, nb_NO, my_MM, sl_SI.

Luxembourg is .lu, but the language code is LB. It causes all kinds of funny bugs when people set their language code to 'lu' and end up with things like date strings automatically translated to Luba-Katanga, a very melodic and completely different language spoken in the Democratic Republic of the Congo.

.sv is El Salvador; both SE and SV were assigned in the same 1974 standard.

Yeah, and as a language code, "se" refers to Northern Sami.