More things

frontend/app/utils/languages.ts

@@ -35,6 +35,22 @@ export function languageName(code: string): string {
   return LANGUAGES.find((l) => l.code === code)?.name ?? code.toUpperCase()
 }
 
+/**
+ * Which language version to open an article in when the address asks for
+ * none: the reader's own language when the article speaks it, else English
+ * as the wiki's common tongue, else null — nothing to pick, so whatever
+ * version already stands for the article answers.
+ */
+export function preferredLanguage(
+  available: readonly (string | undefined)[],
+  reader: string
+): string | null {
+  const codes = new Set(available.map((c) => (c ?? '').trim().toLowerCase()))
+  const wanted = reader.trim().toLowerCase()
+  if (codes.has(wanted)) return wanted
+  return codes.has(DEFAULT_LANGUAGE) ? DEFAULT_LANGUAGE : null
+}
+
 /** The first language no version of an article has been written in yet. */
 export function nextFreeLanguage(taken: readonly string[]): string {
   const takenCodes = taken.map((c) => c.trim().toLowerCase())