How can a man who claims to be a samurai, not even know Japanese. He tells the little girl he only knows English. Seems to me he would learn the language of the Samurai. Someone already mentioned the whole training like a Samurai, but only using guns.
How do you know he only spoke English? I wouldn't place too much credence in his statement to the girl that he only spoke English - or perhaps he was being disingenuous saying he did not "speak" any other language - as he didn't seem to have any difficulty understanding his French-speaking Haitian friend when he told Ghost that the child had returned and had been looking for him.
"We are all in the gutter, but some of us are looking at the stars..." Oscar Wilde
The ice cream man isn't Haitian: Haitians speak Kreeyol. He speaks French, with a beautiful West African inflection. The way he and Ghost Dog repeat everything in parallel shows an understanding and sympathy outside of speech. It's not exactly funny, but it is Jarmusch-standard droll humor.
And why shouldn't Ghost Dog learn Japanese? It's the gateway to the culture he worships. After all, samurai didn't just follow the Bushido code; they also perfected the tea ceremony and wrote poetry. It's a rich tradition to aspire to, not just blind loyalty and cold killing.
I think you're kinda missing the point. Ghost Dog is following the philosophy of the samurai. You don't need to know Japanese in order to be in a samurai mindset. The Hagakure explains Ghost Dog's mindset perfectly. As in for his training, Ghost Dog was more determined to succeed in his methods of execution. The choices of weapons didn't really matter. Besides, he was going up against a bunch of mobsters. It would've been foolish to bring a blade to a gun fight. But I gotta admit, I was bit let down that Ghost Dog never used a katana to perform at least one execution.