Use GNOME's l10n infrastructure - Damned Lies #38
Description
Now that exercise-timer is part of GNOME Circle, you might want to check out Damned Lies - the official translation platform for all things GNOME. Here is an application form with some more info on how to get set up. Actually, it says that as the app is part of GNOME Circle, you don't need to fill out that form. In that case, maybe contact the people that accepted you in GNOME Circle, they might know how to proceed from here.
At Damned Lies, there are many teams from all around the world translating software for GNOME. It's definitely worth checking out!