wrong get_next_posts_page_link creation

[piffpaffpuff]

hi, i have woocommerce installed that creates a custom post type and taxonomy (product and product-category). when i use get_next_posts_page_link or get_previous_posts_page_link the generated url looks like this:

http://localhost:8888/?lang=en/product-category/age/0-24-monate/page/2/&lang=en

well the /?lang=en/ feels some kind of wrong, isn't it? without qtranslate-slug enabled the url is correct:

http://localhost:8888/product-category/age/0-24-monate/page/2/?lang=en

now, is this a bug or a problem with my config? i've tested it with other basic custom post types and the issue persits. my settings: i have permalinks set to postname, and qtranslate language option to query mode. all plugins are updated and wp is on version 3.5.1.