i18n tweaks (#17865)

* Add plural variant to i18n selected key

Weblate needs both keys to populate other languages

* move smart capitalization logic into language provider

* fix key

web/src/context/language-provider.tsx

@@ -45,6 +45,9 @@ export function LanguageProvider({
   }, []);
 
   useEffect(() => {
+    // set document lang for smart capitalization
+    document.documentElement.lang = language;
+
     if (language === systemLanguage) return;
     i18next.changeLanguage(language);
   }, [language, systemLanguage]);