Multi-language support validation (de/en)

Description

The i18n/language system exists in shared/common but has never been fully validated across all player-facing strings. For release, all HUD text, titles, chat messages, and command responses must be available in both German (de) and English (en).

Acceptance Criteria

• All player-facing strings in the server module use the i18n system (no hardcoded English strings)
• lang/en.json and lang/de.json exist and are complete (no missing keys)
• Language is selected per-player based on their Minecraft client locale setting
• Fallback to en if the player's locale is not supported
• CI step validates that en.json and de.json have identical key sets (no missing translation)
• Native German speaker reviews de.json translations for correctness

Technical Details

• LanguageService in shared/common handles locale resolution
• Player locale is available from the Minestom login packet
• CI validation: a simple Gradle task that diffs the key sets of all lang files

Dependencies

• All other feature tickets must be complete first (strings can only be finalized when features are final)

Estimate

M

Start with the LanguageService in shared/common and inspect locale data from the Minestom login packet. Review player-facing strings in the server module and compare lang/en.json with lang/de.json, then define the Gradle key-set validation. Done means supported players receive complete German or English text, unsupported locales fall back to en, and de.json has native-speaker approval.