frontend_func: fix to enable gettext in 'snapmergepuppy'

Additional commit, related to: https://github.com/puppylinux-woof-CE/woof-CE/pull/325
Hmm, gettext works fine only when running 'snapmergepuppy' alone, from terminal, but not via "Save" icon.
It's also necessary to add 'LANG=$OLDLANG' in frontend_func, before calling snapmergepuppy, just like it is with yaf-splash a couple of lines earlier.

Greetings!