We add new method set_user_locale to render the page
with the user's preferred locale.
Note that we add a condition 'if params[:locale].blank?'
to recover the user's preferred locale. This is necessary
because it may be the case that the user does not have an
associated locale, and when execute '@user.locale' when
this value is 'nil', by default returns the default locale.
As we do not want this to happen and we want the locale we
receive as parameter to prevail in this case.
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