Steps

1. Get the language code for the language you want to translate from here. For eg: its en for English, es for Spanish, etc.
2. Create an .arb File: In the lib/l10n/ directory, create a new .arb file named intl_<language_code>.arb. For eg: if you are contributing Spanish, then your filename would be intl_es.arb.
3. Translate Messages: Refer to the template file intl_en.arb as a reference. Inside this file, you will find key-value pairs where the keys should remain the same, but you should translate the corresponding values into your target language.
4. Install Flutter-intl VSCode extension for autogenerating files