Admittedly, my French may be a bit rusty here, but what's the difference to, e.g., FourSquare or Facebook's Check Ins?
@th0br0 J'arrive is absolutely not a check-in like app.
It's an app that going to help you to share your location with your friends for a given amount of time. You can compare it with 'Find my Friend' but available for everyone (not only iPhone's user).
And don't worry about your French, J'arrive will be available in english (with of course some french touch magic in it ;)).
We have just updated our website, it's in english now ;).