Как лучше всего перевести на английский слова «гопник» и «быдло», чтобы иностранцы поняли, что это?

There's less mystery to Russia than most Russians think. Гопник is not that hard to translate, really.

To a Brit it's a chav. Not only does this word match the meaning of the Russian noun - both words appear to be backronyms, too!

To a Scot it can be a ned, though this word has more of a criminal connotation.

And if you're talking to an American or someone else unfamiliar with the terms above, you'll simply have to explain the concept of гопник in a few words. Personally, I'd go with "urban white trash."

No need for flowery language.