The language is not a problem, because in any firmware there are all the languages.
The country isn't so important... just some differences about the kind of apps that are preinstalled.
Instead the main difference is with the brand: there's always all the stuff about that brand and sometimes (and just sometimes) you might have some issues with configuration of apn and wifi.
The best thing is to choose an unbranded rom and if you can't, try a branded one
Generic HK
What's this device? I never heard about that...
Docomo is from Japan, maybe you should have a look at Japanese forums (if you can read)