Mainsail is available in many different localizations translated by the community.
To change the language, go to the "Interface Settings" by clicking on the two gears at the top right.
In the "GENERAL" section, select your preferred language.
You are welcome to help us improve the translations. You can find instructions on how to do that over here.
Get in touch with us on our Discord server. We will assign you a translator role for your language and inform you in advance when a new Mainsail version will be released, so that you can make the changes.