Surprisingly, I stumbled upon a solution in the meantime though I can't explain why it works.
I tried to use the function pll__ in the gallery template file with the problematic strings, and it solved the problem. This is how it looks in the gallery.php file:
<?php echo pll__($gallery->title) ?>
So, it seems that some variables need this, some don't, and they vary from page to page. Maybe you can tell me why this is the case.
Anyway, many thanks for the help, and for the great plugin you created!