Phrase labels are not validated leading to errors

We should add restriction to "Labels" so ONLY "alpha-number" symbols can be used. In some case I saw users putting "spaces" and other non-latin letters which actually messes up during the Import. Also validate, that phrase starts with "lu_" or "la_".

I propose we tell admin (during phrase adding/editing), what other phrases in language pack have same translation as it's phrase. In most cases duplicate translation will mean nothing, but sometimes it will help to prevent duplicate translations. Validate by CRC32 of translation.