[TYPO3-core] Translation 'en' missing

[Daniel Alder]

Please review my patch:
https://review.typo3.org/17414

For this who still are skeptic:
- BE: default = EN -> i am fine with that
- FE: in l10n mode i can't translate 'en'. and i don't want to use TS or 
use hooks. just add an 'en' translation in typo3conf/l10n like i used it 
since ever.

additional info for TS behavior:
http://forge.typo3.org/issues/42084#note-7

if you want to support both 'default' and 'en' feel free to add a 
fallback for 'en'.

but i think it's confusing (devs setting 'en' in config but have to use 
'default' in TS, makes no sense to me), not strict (it's an exception 
for 'default' only, hardcoded and will cause problems when trying to 
standardize language handling in future)

so please review this patch.
thanks in advance!