Manual testing is not need - covered by automatic tests. If for whatever reason human testing was required: Login as admin Navigate to Language > Language packs in site administration Install Cymraeg (cy) & Dansk (da) language packs Navigate to Language > Language settings in site administration Set Languages on language menu (langlist) to: "en|English,cy|Welsh" and save changes Confirm language menu now shows [ Welsh | English ] Set langlist to: "en|English,xx|Welsh" and save changes Confirm you see the error "Invalid language code: xx" Set langlist to: "en|English,xx" and save changes Confirm you see the error "Invalid language code: xx" Set langlist to: "xx|Welsh" and save changes Confirm you see the error "Invalid language code: xx" Set langlist to: "xx" and save changes Confirm you see the error "Invalid language code: xx" Clear langlist field and save changes Confirm language menu now shows [ Cymraeg (cy) | Dansk (da) | English (en) ]