Polish support language isn't available yet.
Indeed, if you can do it you're welcome.

Language files are in "language" directory, and only there.

You can translate only a subset of files (iso.txt and common.lang.php at least), see what I've just replied to Pavel Budka the translator of Czech language files for release 1.5.0 :

z0rglub by email wrote:

"Pavel Budka" <pab@mail.cz> writes:

> Attached czech translation is based on english one (en_UK.iso-8859-1).
> The translation corresponds with a version PWG 1.5.  The translation
> is in attached zip file is (cz_CZ.cp-1250).  Just files
> common.lang.php and iso.txt are changed (localized) everything else is
> the same as in en_UK.iso-8859-1.

Languages management has been improved in branch 1.5, so that you don't
have to copy all files if you translated only a subset of language
files. In the archive you gave me, I only kept the two translated
files. PhpWebGallery finds the right language page by itself :-) (see
function get_language_filepath)

remik let's do the polish lang for that gallery - please write my gg72556 to work with that & U bizon better drop that idea