Im beginning with boost_locale too, here is how i make it work...
First make sure your .mo file is named exactly as the domain your refering to, in this case hello.mo
Put the .mo file into the correct file structure, for example if your trying to translate to spanish this would be ./es_ES/LC_MESSAGES/hello.mo
Make sure you instantiate the global locale like this, locale::global(gen("es_ES.UTF-8"));
Hope this helps.