Spec the i18n pass for an existing React app

Your React and TypeScript app is English only, with about 1,200 hardcoded strings. You are launching in German and Polish. A coding agent will do the internationalization (i18n): move the strings into message catalogs, wrap them with react-i18next, which the repo already has, and handle plurals, numbers and dates.

Write the spec you would hand the agent.

What you know:

• translators work from JSON catalogs, one per locale, and never see the code
• the app shows counts such as "3 files", prices in euros and order dates
• some strings are built by joining pieces, such as "Welcome back, " + name
• product names come from the API and must not be translated

Then, in a few sentences: what does "add i18n to this app" get wrong?