Add backend internationalization for responses

The strings in the backend are all hardcoded to English and are also repeated in many places, meaning that if you want to change one message you have to change all occurrences of it.

Adding internationalization is not only a way to add more user friendly responses but it also adds that unification of messages.
It could be done similarly to how it's being done in the frontend.

The email notification messages are also all in English but since they are a bit more specific and have a different way of being handled (with jinja templates) they can be done separately in a future issue, they don't need to be included in this one.

No backend files or tests are named. Start by reviewing how internationalization is handled in the frontend and inventory repeated English backend responses. Define the backend response scope, keeping email notification messages rendered by Jinja templates excluded for a future issue.

Written by the indexing model from the issue text.