Let's just be clear: it is racism, but it's just the sort of naturalistic —more comfortable with what you know and like— discrimination that's subconscious and extremely hard to avoid.

One would hope an investor would be able to look past that, but when so much about pitching is already about making your audience comfortable, you'd be silly to ignore it. If your co-founder CAN do the talk and is likely to be more "familiar" to your audience, you probably want them to do more of the talking.

Also remember that it's not just who's the native speaker. I'm AWFUL at public talking and I've been speaking the bloody language for 30 years. Confidence and passion are important.

Without seeing you both pitch, I can't tell you what to do, but you could tell friends and colleagues how important this is and try to get their honest opinion.