Add German locale

This commit adds a German translation for the add-on, and fixes the
description in the English one.

src/_locales/de/messages.json

@@ -0,0 +1,24 @@
+{
+  "extensionName": {
+    "message": "Geburtstagskalender",
+    "description": "Main add-on title"
+  },
+  "extensionDescription": {
+    "message": "Fügt Kalender zu Thunderbird hinzu, die die Geburtstage aus dem Adressbuch enthalten.",
+    "description": "Main add-on description"
+  },
+  "defaultCalendarName": {
+    "message": "Geburtstage in $addressbook$",
+    "description": "Default name for birthday calendars",
+    "placeholders": {
+      "addressbook": {
+        "content": "$1",
+        "example": "Personal Address Book"
+      }
+    }
+  },
+  "addressBookSelection": {
+    "message": "Adressbücher auswählen, für die Geburtstagskalender angezeigt werden sollen:",
+    "description": "Label above the list of address books in the preferences"
+  }
+}

src/_locales/en/messages.json

@@ -4,7 +4,7 @@
     "description": "Main add-on title"
   },
   "extensionDescription": {
-    "message": "Adds calendars containing the birthdays from your address books",
+    "message": "Adds calendars containing the birthdays from your address books.",
     "description": "Main add-on description"
   },
   "defaultCalendarName": {