Doesn’t update language when I select another one in the language dropdown

@Tacsipacsi: I've submitted gerrit:745997 to add a hook when the language changes, which should hopefully make this a lot easier. I've also made some preparatory changes to how the processSourcemessage() function in the script works, so that it shouldn't be a problem to process the source message even after it has been altered. (This was also done to make the script update the markup immediately after saving a change instead of just after a reload, as mentioned in the other thread.)

This issue should be solved now, Tacsipacsi! Feel free to test it out. :-)