Just to let you know, I tried out the last piece of advice I gave you on UM 16.04 Alpha and it worked to a certain degree, the only thing that didn't work was the fact that I could not find the new locale file afterwards, it placed it somewhere else but I cannot find it!.
I had English and German on my system and changed it to German only!:
I hope you will give it a try and if things go wrong, you can always do a fresh install and keep your data intact if you follow this advice here (DO NOT FORMAT YOUR HOME PARTITION!):