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Looks like an upside down Serbian flag but I ain't no flag expert.
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It is a Serbian flag. I think in the photo it's tangled up or in high wind or something. It could be just upside down, the Populist Party (russia supporting) just won another election.
https://www.independent.co.uk/news/world/americas/us-politics/aleksandar-vucic-ap-serbia-russia-europe-b2050335.html
Maybe the dude doesn't like what is going on back in the home country.
If it's actually right side up there are several it could be, Slovenia among them.