Add english to list of locales to pick

Useful to make F-Droid be in English if the system language is
different.

Fixes issue brought up in https://f-droid.org/forums/topic/f-droid-0-97-language-selection/

F-Droid/res/values/donottranslate.xml

@@ -25,6 +25,7 @@
 
     <string-array name="languageValues">
         <item></item>
+        <item>en</item>
         <item>ar</item>
         <item>ast</item>
         <item>bg</item>

F-Droid/tools/langs-list-check.py

@@ -9,7 +9,7 @@ import re
 from xml.etree import ElementTree
 
 prefs = set([''])
-trans = set([''])
+trans = set(['', 'en'])
 
 donottranslate = os.path.join('res', 'values', 'donottranslate.xml')