Fix invalid enumeration string value display

Prior to this, an invalid enumeration string value (e.g. @32@) was
displayed if the corresponding localized value did not exist.

This allows fallback to English language for individual enum elements.

Fixes #16975, https://github.com/mantisbt/mantisbt/pull/198

Signed-off-by: Damien Regad <dregad@mantisbt.org>