There are certain bars and such where you can get Japanese girls easily even if you don't speak Japanese at all. Those are the places where the "gaijin hunters" go to find a cool foreigners to have sex and learn English with. BUT. In my opinion, the level of those girls isn't that high. And they always seem a little crazy or something. I don't know. By learning Japanese you can start a conversation with anyone. If you're good looking you can easily give it a go with those Tokyo Drift kinda girls and even if you're just a normal guy you can find pretty awesome girls because there's a lot of Japanese girls who are genuinely interested in foreigners but they don't have what it takes to communicate in English...
Know the language is helpful.
In fact 99% those Asian girls appear in the bar to date foreigners are lower than average looking, either prostitutes or unfavour to local Japanese males so they reluctantly approach foreigners for purpose (also the same situation in other Asian cities like China, Thailand etc.), its difficult to see fine looking East Asian woman would date foreigners in reality unless escort service hookers.
For hookers, there're rarely emotional attachment involved. Have seen a dark looking bar girl in an internet cafe chatting with 10+ western males concurrently by Skype, copy & paste the same message (e.g. I love you long time etc.) to every men on her list, very funny