Don’t require the student to have knowledge of the country’s language? I have found quite a few Germany universities that offer sociology in English, however, they still require for the applicant to demonstrate a “very good level of German” or in some cases, at least enroll in a German class during summer.
I understand the practicality of this but I am still curious if my mentioned requirements can be met 
Even those that are “tuition-free” are not tuition free for non-European citizens.
Actually, that’s not the case in Germany, Irlandaise, or I think Norway or Finland. Tuition is free for anyone admitted to state funded universities, regardless of their citizenship or residence status. You still have to fund living costs, of course. Also, bear in mind you’ll need to apply for a student visa which may or may not allow the student to work.
The main barriers are (a) language, and (b) academic requirements. In the case of Germany, I believe it can be quite tough for those not educated in the German system to meet the required standards.
This question is too broad and open. Start looking at Germany, then branch out to others.
I would suggest that you inquire directly with the German universities in question, or those with informed answers, such as the German embassy in the U.S., to start with. I am aware that some policies regarding German universities have changed recently; however, the poster who states that non-European Union citizens are charged tuition – and a much higher amount at that – is generally correct, except perhaps Norway, and now Finland. It is a tricky area that requires due diligence.
I also would not suggest that any student attend a German university without an intent to take at least some German courses while there. And I wouldn’t recommend just going for sociology because it might be in English; there won’t be great career prospects with a degree like that, and the top sociology departments – Berkeley, Wisconsin, etc. – are located in the U.S.
There are departments in Germany that will have English-based courses; Anglistik (like our “English” departments), perhaps some courses in Business Admin. (that appeal to international, BBA-type students in a lingua franca), etc. Again, just going to Germany to study English isn’t the best solution in an attempt to save some tuition money. The undergraduate degree may not provide great career prospects in an already-saddled European economy, which will almost universally favor job applicants with EU status (read: UK citizens for any job that requires native English fluency). Also, with a European degree, you will need a way of finding residency status thereafter to stay on. And in the U.S., those with solid U.S. university degrees will likely be favored for most job openings.
This is not to discourage attending university in one of those countries, as in principle I think it could be wonderful. But caveat emptor: know what you are getting into, both during college, and thereafter.