When user select different default language than English and if in that language pack is variable which is not in English language pack than in ACP - Customise - Language packs - British English (Language pack details) is message "The following language variables are missing from the “British English” language pack"...
It can confuse users who believe that there is something wrong with English language pack, but problem is with their default language pack. I believe it would be better to check variables in language packs against English (if it's installed) and not against default language. Short discussion about this issue is here.