in my opinion there are three things related to the issue:
1.) The old behavior to fall back to the default language was broken by changing the language names from long names to short names e.g. de_DE to de, en_US to en and so on. The menu items for "right click on tree items" were always in english as the string was not in the translation file and the fallback to the en_US worked fine. I supplied a small fix to establish another fallback level which will look for the default language with the short name.
3.) There seems to be a lot i18n function calls without a default/fall-back string, which is the last level of fall back when looking for a translation. These function calls should always include a default string (english is better than an empty field...).
I am not sure if I can find the time to search for all calls of the i18n function and add a default string to all of them if they do not already have one.