You can open in English if you want, but depending on how international it is, it'll be harder to find girls interested in English speaking guys.
If you can talk about: food, fashion, drinking, music, or travel, (in either language) you'll be pretty set. I've been on dates where the girl, despite learning English, was too embarrassed to speak it until much later. On the plus side, it'll force you to work on conversational Japanese and get better.
If she starts talking really fast, maintain your frame as the leader, and tell her she needs to repeat what she said and slower. Depending on how far you're into the conversation, you could incorporate a playful touch when you tell her to stop and repeat because she's speaking too fast.
If you can talk about: food, fashion, drinking, music, or travel, (in either language) you'll be pretty set. I've been on dates where the girl, despite learning English, was too embarrassed to speak it until much later. On the plus side, it'll force you to work on conversational Japanese and get better.
If she starts talking really fast, maintain your frame as the leader, and tell her she needs to repeat what she said and slower. Depending on how far you're into the conversation, you could incorporate a playful touch when you tell her to stop and repeat because she's speaking too fast.