Be polite and classy
What to do to please your Japanese girlfriend?
There is an old Samurai saying that says that “even messages delivered to your enemy have to be delivered in the politest way possible”.
The Japanese society is very concerned with politeness in every form, including talking politely. Your Japanese date can be offended fairly easily if you are not using the right word or showing the right amount or courtesy.
The Japanese people probably say "thank you," “excuse me” and “I’m sorry” more than any people on the planet.
Westerners, especially Americans are very fond of using the f-word but don't swear. Ever. Don't speak in a vulgar matter. It is crude and impolite to be vulgar - not the etiquette of a gentleman.
By speaking politely, you show the level of your character and inner development, and this is something she won't resist.