When should I put translations in a Tastic configuration and not in the code?
This depends on whether you want your Frontend Manager to be able to edit these values or not.
Where you have a field type of string available in your Tastic configuration, that's translatable by default. This means your Frontend Manager can change the translations within the Frontastic Editor.
If you have content that's more static and it won't often need to be translated, for example, a button that says "OK", then this should go into the code.
React-Intl for this and it's where you can use the
FormattedMessage component. Our
DependencyInjector gives you the ability to overwrite our default
App.IntlProvider, see here.