For Gallery 2.1:
http://yourdomain.com/gallery2/main.php?g2_language=fr
For Gallery 2.0.x:
http://yourdomain.com/gallery2/main.php?g2_controller=core.ChangeLanguage&g2_language=fr&g2_return=main.php
These URL select French and then display or redirect to the top album of the gallery. Set the g2_language and g2_return values for the language and redirect target for your URL.