All localizations are hosted on Crowdin.
Logged in with your Crowdin account, open the Crowdin project, select the language you want to improve and use the "Join" button.
Create an issue mentioning which language you would like to be added.
To submit a script that converts an export from an other application to the format used by Material Notes, either open an issue and provide the script, or open a pull request that add your script to the external_imports directory (in that case, please name the file after the application).