"By default, Mastodon will automatically detect the visitor’s language from browser headers and display the Mastodon interface in that language (if it’s supported). If you are running a language-specific or regional server, that behaviour may mislead visitors who do not speak your language into signing up on your server. For this reason, you may want to set this variable to a specific language."
This explanation is copied from the Mastodon documentation for Default Locale, and on that page, you can find a list of available languages that Mastodon supports. If you can find the language you want to set your server, please email the language code with your request to change your server's Default Locale to email@example.com.