Minor I have one language installed, e.g. "English" (id = 1). Then I create several products (because their name/description is multilingual). Then in database I could verify, that such products have values in "l1_Name", "l1_Description" fields, but doesn't have values inside "l2_Name", "l2_Description" fields, because I don't have language with ID = 2 yet.
I add new language to the site, e.g. "Russian" (id = 2). Products I've created still have empty values for "l2_" fields, because kMultiLanguage formatter class will automatically show values from l1_ fields, when requested l2_* field value is missing.
When I try to sort products in category on Front-End, when I'm on Russian language (id = 2), then it doesn't sort, because l2_* fields for products created before "Russian" language was added is empty.
I propose we use same kind of functionality for multilingual field ordering as kMultiLanguage formatter class: sort value on current language, when present, but when not present, then sort value in primary language.