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A very good question that probably many webmasters have faced or will face.
>>buy differents domains for each language (then a .com, a .de, etc...) and submit it to the different "national SE" but from the homepage of the centralwebsite (the .com one) the links given to the different languages redirect to subdomains and not to different domains...
what do you think is better?
If you have the budget and the language capabilities (not to mention the geographic necessities for some top level domains) then I would suggest that you go the many domains route. Not only will you get a "home page" boost for each language, but you can also cross link each different domain and increase your link popularity and themes value.
I really can't see any downside to having different domains for each country provided you have the necessary resources to carry it out.