To delete a translation, a link has been added. This
link works for the selected language. It hides all the
things related to a language (the tab and the text_area)
and empties the text area, so that the value is blank
in the param hash. A variable called `delete_translations[]`
is changed.
e.g. If admin wants to remove English language,
delete_translations[:en] will be 1; if not, it will be 0.
When the milestone is updated, there is a before_action
callback that cleans the selected languages for deletion
(looking the delete_translations[] variable).
Because of the deleted translations are blank in param hash,
them won't be saved in DB.
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