Add 'mode' and 'game_mode' translations to German

[PilkeySEK]

Please complete the following:

• I have added screenshots for all UI updates
• I confirm I have thoroughly tested these changes and take full responsibility for any bugs introduced
• I understand that submitting code with bugs that could have been caught through manual testing blocks releases and new features for all contributors

Please put your Discord username so you can be contacted if a bug or regression is found:

PilkeySEK

Before vs. After (there is no good translation for "FFA" and "Teams" is the same):

[evanpelle]

thanks!