Removing word "Medias" from translations

According to http://www.oxforddictionaries.com/definition/american_english/media the word "media" is the plural form of "medium". Therefore, there is no word "medias".
I did not go so far to rename the models and associations, but IMO at least the translations should follow grammar.
Merge pull request #459 from gedsed/patch-1

Removing word "Medias" from translations