lang.js file gets overwritten on Update

Hello,
we made a few changes to our lang.js file, because some translations were missing for our needed language(germany). 
The problem is that every update overwrites the existing file and therefore all our changes. 
The only thing I found in the forum, was that you could submit your own lang.xls so it will be added for everyone on the next update.
But is there another way? Because we are not totaly confident that our translation is 100% accurate with its original meaning and also if its the same with translations of the same word in other parts of the software. Therefore we would not submit it if there is another way to prevent the lose of our changes.

Thanks in advance

T. Schmidt

Thanks but I am afraid that is not an option for us.
Therfore we are now trying to change the translations in the lang.xls file and then submit it.
The problem we are facing with that is the translations that are missing do not show up in the file.
In the file (downloaded 10min ago) there are only 3 translations missing but none belongs to the missing ones we want to fill.
So where can we change them when they are not in the lang.xls file?