Revert "Don't symbolize tainted data." [ci skip]

Reason: i18n whitelists now locales without passing through symbols,
see https://github.com/svenfuchs/i18n/blob/master/lib/i18n.rb#L278.
Therefore, this snippet is no longer a good practice.

This reverts commit ec0664a6eb8906fcd31a53a1efad69bdc7fe6f5b.