I've just noticed that if I add ?lang=ru at the end of the URL of each post or custom-post, the post is still displayed in the Romanian (default) and the rest is in Russian (even though the post in not translated). This is close to what I want if the post is not translated.
Demo:
http://bikeportal.md/results-2013-1/
http://bikeportal.md/results-2013-1/?lang=ru
However, this doesn't work with categories and other taxonomies + the switcher still wants to redirect me to the homepage (doesn't add ?lang=ru in the href).
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This still has little-to-nothing to do with my main problem: the translated categories and other taxonomies are empty because I can't assign untranslated posts to them.
Demo:
http://bikeportal.md/category/results/
http://bikeportal.md/category/results-ru/
-> As you can see, the second one is a translated category of posts and it's empty -> thus the language switcher from default language sees it and doesn't add the correct URL to the Russian category.
Any suggestions, tips, codes, idea would be very appreciated! I really don't want to switch to that heavy WPML...