Provide localised language versions
IMO, HIBP is so useful that every single person in the world should have it bookmarked and all companies should monitor their domains accounts using it. Some users in our company use their business email address to create accounts in several websites, and thanks to HIBP our IT team is warned when one of them is pwned.
We thought it would be a great idea to tell everyone about HIBP so they could verify and monitor their own personal accounts, so we did it by sending an email telling about HIBP to everyone in the company. Everyone was able to check their personal email address, but a lot of users came to us asking how they should proceed to register to be warned by HIBP when their addresses appears in a breach.
"Notify me" is effective, even for people that don't speak English, but we noticed certain difficulty for a considerable number of people (we speak Portuguese in our country).
I'd like to suggest the translation of the site and the alerts it sends to the users.
HIBP offers an invaluable service to the internet, for free. I'm sure that people will help translate it to their languages for free too (l10n). I'm one of those. Count me on when (and if) you decide to make it available in other languages.
Jose Luis commented
Donde puedo cambiar el "Lenguqaaje" al Español? Gracias!