Hello,
unfortunately, this subject has never been fixed and commented by the author within the last 6 months or more.
Polylang itself works well on the whole site except when it comes to NGGallery. The problem is that sub-galleries are shown in the main language but as soon as you try to navigate them in another language, the site redirects to the main gallery page.
Check yourself: http://www.riverside-kustomz.de/en/gallery/ (default is german)
Both plugins are always up to date.
Never worked for me since I installed version 1.5