I'm not sure about actual majors at BU specifically (I believe it can matter at specific other schools, but I haven't really heard of anything like that here) but the acceptance rate sometimes does vary between colleges. CFA (college of fine arts) for example has a wildly different acceptance rate because it has a different application process.

Again, I can't confirm the actual process because no one knows that for certain. I would say, though, that it's more likely to matter between colleges, but even that is just speculation on my part. Also, a crap ton of people come into the school undeclared or switch immediately, so it's never going to be the be all end all of your application even if the acceptance rate varies by a small amount between majors. They may try to admit more students to one major and not another to shift resources around or develop some departments, but it's unlikely that you'll be able to find out how that will affect you specifically at any particular school, BU included.

The far more important part is that you apply for something you've demonstrated interest in and can prove to the college you have some reason for wanting to study. If you've never taken a japanese class and can't write a good essay explaining why you care about it, it probably wouldn't be a great choice, for example.