Add 'mode' and 'game_mode' translations to German (#364)

## Please complete the following:

- [X] I have added screenshots for all UI updates
- [X] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [X] 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):

![image](https://github.com/user-attachments/assets/8fdbeee1-4d39-4846-9553-3d1de96e22a9)

![image](https://github.com/user-attachments/assets/91fb678f-f2fb-49fb-9619-785d045013a2)

resources/lang/de.json

@@ -138,6 +138,7 @@
     "title": "Private Lobby",
     "map": "Karte",
     "difficulty": "Schwierigkeit",
+    "mode": "Modus",
     "options_title": "Optionen",
     "bots": "Bots: ",
     "bots_disabled": "Deaktiviert",
@@ -156,5 +157,9 @@
     "Balanced": "Ausgewogen",
     "Intense": "Intensiv",
     "Impossible": "Unmöglich"
+  },
+  "game_mode": {
+    "ffa": "Free for All",
+    "teams": "Teams"
   }
 }