There are json files in src/lib/translations for each language. You can add a new language by creating a new file, and adding it to the src/lib/translations/index.ts file.
In the svelte page/components, make sure to call useI18n() once at top level to set the correct locale for SSR (which can have parallel requests in different locales).
Install the i18n-ally extension for VSCode, which will help you with the translations, including finding missing translations.