Theme
Switch between Light, Dark, and System (follow iPhone’s appearance) under Settings → Appearance. The change is applied immediately.
The full-screen barcode view always uses Light mode internally, regardless of the app theme — that’s intentional to keep the barcode against a bright white background for cashier scanners.
Language
English and Polish are bundled. Pick one in Settings → Language, then tap Restart in the prompt. Scandio quits and the next launch is in your chosen language.
If you prefer to follow iPhone’s system language, leave the picker on the locale that matches your device — Scandio falls back to English for any system language other than Polish.