>I'm from Berlin-Brandenburg area.
That doesn't tell us much. Either you live in one of the most cosmopolitan and socially libertine places of Europe or some post-soviet backwater village in one of Germany's most desolate areas. If I give the advice to "go to a bar and talk to people", you will have wildly different experiences depending on where you do this. Although I guess it is one of the most straightforward ways to get some social interaction.
>I doubt that smiling is crucial, but I'll try.
I haven't met you in person, so I don't know. Maybe something else is the problem or you are surrounded by idiots and do everything right yourself. It's just a cultural difference you might want to be aware of. You might think "smiling is not important" and a German looking at you might think "why does this Russian look like he wants to kill me".
>I feel like Germans are kind of afraid to know eachother a little bit more. So on the surface level I don't pass the vibe check, because I am just a strange guy, and nothing is going to be further.
Very insightful. I think this is true to some degree for most Germanic cultures, in some areas more, in some areas less. Within Germany there are regional differences. Again, especially in rural areas you might have a problem while some Berliners might be curious about strangers. If you're in a more closed community in a little village, I recommend you check out "Vereine" or "Volkshochschule" or some other activity or group like that. Maybe there is something targeted at foreigners for practicing German, too. If you have a common goal or activity, people will be more open.
>I also feel that it is like a taboo, to get acquainted with somebody without some innocent reason.
Yes. You need a reason, otherwise it gets weird easily.