As other members have said, 'jolly' is only used in England in this way, and it definitely has old-fashioned and slightly upper-class associations. Personally, I do use 'jolly' as an intensifier, sometimes in a slightly jokey way, because I'm conscious that it's very 'English' and somewhat outdated. Is it weird? It depends who you are. It's not weird for me or for many people that I know. But it would probably be strange for a Scotsman, an Irishman, a Welshman, an American, a Canadian, an Australian, a South African or a New Zealander to say 'Jolly good'. And it would almost certainly be odd for you, as a Belarusian to say this. Non-native speakers of a language are best advised to stick to standard phrases rather than regional variations.

I wouldn't try to use it. It's definitely British. It's one of a number of expressions that we in the United States think of a "characteristically British" (which usually means "outdated.") Also, UK speakers might correct me, but I don't think that "jolly" and "very" are interchangeable. In fact, your example sounds wrong to me. "Jolly good" expresses an enthusiastic admiration of something. "That's a jolly good book." "Arsenal is a jolly good team." "Oh, well played, jolly good show." I don't think you can express admiration for how good you, yourself feel. I'm sorry if that isn't clear, but it's such a weird idea that I don't really know how to say it. Using "jolly" in place of "very" won't make it easier or more comfortable for English speakers. It might sound odd. It might make your listeners think "has she been reading P. G. Wodehouse?" (He was a humorous writer whose characters speak in exaggerated Edwardian upper-class informal English).

First of all, it is something that you would only here in England and it is a little bit old fashioned now. I would also associate this word with upper-class England. That said, I wouldn't recommend that you use it in conversation. It is better to practice using more current English in my opinion.

It's rather British! :D Some words and expressions are regional. "Jolly" (in the sense of "very") is one of them.