Add internationalization (multiple language) support

Add internationalization support so the UI can be translated into multiple languages.

Proposal / acceptance criteria:
- Extract user-visible strings to a resource system (gettext, Fluent, or similar) with fallback to English.
- Provide tooling or instructions for adding translations and contributing language files.
- Make language selectable at runtime and/or follow system locale.
- Start with scaffolding and include at least one non-English translation as proof-of-concept.
- Ensure build/runtime size and performance impact is minimal.
- Make sure it works with steam's internationalization support

No files or tests are named. Start by mapping the UI entry points and user-visible strings, then inspect the build/runtime integration points and Steam's internationalization support. Done means English fallback, translation tooling or instructions, runtime or system-locale selection, one non-English proof-of-concept, and minimal size and performance impact.

Written by the indexing model from the issue text.