Maybe just try to merge your po file with the new one using msgcat:
http://stackoverflow.com/questions/8816646/what-is-msgcat-and-how-t...

Hello

I've just finished a tool for such problems.
You can download it from here:
https://github.com/mlocati/concrete5/raw/i18n/i18n/i18n.php...
or browse here for knowing what's going on:
https://github.com/mlocati/concrete5/tree/i18n...

What you have to do is:
1) copy your messages.po file in the folder
/languages/<languagecode>/LC_MESSAGES/

2) launch my tool:
php i18n.php --webroot=<WhereYouHaveInstalledConcrete5> --language=<languagecode> --createpot=yes --createpo=yes --compile=no

Your messages.po will be updated with the new strings (keeping the old ones).

For more operations that i18n.php can do:
php i18n.php --help

For an interactive session (very easy to use):
php i18n.php --interactive

Ciao!
Michele

This should get into concrete core!!!!