It’s a good sample of youth slang. Teenagers use words or cliches borrowed from God knows where (other languages, social networks, computer games, etc) to substitute normal words or phrases, which, as they think, makes them sound cool and trendy. They may do that just for fun, or, even, when they want people from other environment (e.g. parents, grandparents) not to figure out the theme. What is important, when texting, they spell all this stuff in Cyrillic. Some examples from my 17-year-old niece:
«бест френд форева» - “good friend” (mind, here «бест» /best/ is an exaggeration);
«гуд бай май лав» - sort of “see you soon” (again, «май лав» /my love/ is an exaggeration); otherwise «гуд бай май лав» may also mean “farewell” to show you don’t want to continue relationship (it depends, as you can see, on the context);
«аривидерчи» - “bye”;
«дедлайны» - “lessons missed by you at school”;
«браза» - “brother” (either literally or figuratively);
«гоу» or «лэтс гоу» - “(let’s) go”;
as well as «рофлить», «хайпнуть», «лайкнуть», «шарить», «лол», «имхо» and so on to infinity ! (quite many adults also seem to become addictive to this slang)))
In your example slang phrases (1) «братюшка интернэшнл форевер» and (2) «мой бест бойфренд и лав навеки» also sound exaggerated. Of course, young people don't think about things like 'exaggeration' - they just use 'ready-made' cliches borrowed from another context. Moreover, some of them don't even know the exact meaning of the original word/phrase . IMHO, in your case these may simply stand for (1) “friend from another country” and (2) “my dear friend”, and are probably used to show friendliness, or as a joke (it depends).