Difficulty translation key (#1959)

Move the key to name mapping for difficulties into the translation
system.

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

resources/lang/en.json

@@ -250,9 +250,9 @@
   },
   "difficulty": {
     "difficulty": "Difficulty",
-    "Relaxed": "Relaxed",
-    "Balanced": "Balanced",
-    "Intense": "Intense",
+    "Easy": "Relaxed",
+    "Medium": "Balanced",
+    "Hard": "Intense",
     "Impossible": "Impossible"
   },
   "game_mode": {