Hardcoded English as the default; additional languages are separate string files. Proposed layout:
src/i18n/
strings_en_default.rs ← canonical source of all strings
strings_nl.rs
strings_de.rs
strings_fr.rs
...
A language selector in Settings → writes the choice to rustpg.conf. On startup, i18n::load(lang) returns a Strings struct; all UI code goes through it instead of string literals. New strings are added to strings_en_default.rs first; translations follow.
Language list from original request — EU official languages plus a wider set:
Bulgarian, Croatian, Czech, Danish, Dutch, English, Estonian, Finnish, French, German, Greek, Hungarian, Irish, Italian, Latvian, Lithuanian, Maltese, Polish, Portuguese, Romanian, Slovak, Slovenian, Spanish, Swedish;
French, German, Hindi, Indonesian, Italian, Chinese, Korean, Portuguese, Spanish (Latin America), Spanish (Spain), Japanese, Arabic.