Missing languages strings should be detected by the install process while updating the board. I try this for my german translation and it work for me. The string check found the missing one and add it in english. I only have to translate it.

This should work with every translation.

my board: http://www.dragonclan-forum.de
my hobby: http://www.biker-reise.de
Ich kann bei Fragen zu UBBthreads in Deutsch weiterhelfen oder es zumindest versuchen