Internationalization

# Objective

Have an internationalization system.

# Current situation

- The application is not international
- It is hard to contribute on text (typos, vocabulary, …)

# Expected situation

- There is an internationalization system
- Texts are externalized in a file easily usable by non-technical contributors

The issue names no files, tests, or entry points. Start by surveying the application’s user-facing text and how contributors currently edit it, then define an internationalization approach and an external text-file format that non-technical contributors can use; done means both expected situations are met.

Written by the indexing model from the issue text.