Update translations

Actually translate the language dropdown label & add the string.
Remove some unused ones.

src/components/structures/UserSettings.js

@@ -567,7 +567,7 @@ module.exports = React.createClass({
     _renderLanguageSetting: function () {
         const LanguageDropdown = sdk.getComponent('views.elements.LanguageDropdown');
         return <div>
-            <label htmlFor="languageSelector">Interface Language</label>
+            <label htmlFor="languageSelector">{_t('Interface Language')}</label>
             <LanguageDropdown ref="language" onOptionChange={this.onLanguageChange}
                           className="mx_UserSettings_language"
                           value={this.state.Language}

src/i18n/strings/en_EN.json

@@ -283,6 +283,7 @@
   "I have verified my email address": "I have verified my email address",
   "Import E2E room keys": "Import E2E room keys",
   "Incorrect verification code": "Incorrect verification code",
+  "Interface language": "Interface language",
   "Invalid alias format": "Invalid alias format",
   "Invalid address format": "Invalid address format",
   "Invalid Email Address": "Invalid Email Address",
@@ -575,11 +576,9 @@
   "Share message history with new users": "Share message history with new users",
   "Encrypt room": "Encrypt room",
   "There are no visible files in this room": "There are no visible files in this room",
-  "Error changing language": "Error changing language",
   "Room": "Room",
   "Room name (optional)": "Room name (optional)",
   "Who would you like to add to this room?": "Who would you like to add to this room?",
-  "Riot was unable to find the correct Data for the selected Language.": "Riot was unable to find the correct Data for the selected Language.",
   "Connectivity to the server has been lost.": "Connectivity to the server has been lost.",
   "Sent messages will be stored until your connection has returned.": "Sent messages will be stored until your connection has returned.",
   "Auto-complete": "Auto-complete",