lib/api: Correct ordering of Accept-Language codes by weight (fixes #9670) (#9671)

The preference for languages in the Accept-Language header field
should not be deduced from the listed order, but from the passed
"quality values", according to the HTTP specification:
https://httpwg.org/specs/rfc9110.html#field.accept-language

This implements the parsing of q=values and ordering within the API
backend, to not complicate things further in the GUI code.  Entries
with invalid (unparseable) quality values are discarded completely.

* gui: Fix API endpoint in comment.

gui/default/syncthing/core/localeService.js

@@ -74,9 +74,9 @@ angular.module('syncthing.core')
                             }
 
                             matching = _availableLocales.filter(function (possibleLang) {
-                                // The langs returned by the /svc/langs call will be in lower
+                                // The langs returned by the /rest/svc/langs call will be in
-                                // case. We compare to the lowercase version of the language
+                                // lower case. We compare to the lowercase version of the
-                                // code we have as well.
+                                // language code we have as well.
                                 possibleLang = possibleLang.toLowerCase();
                                 if (possibleLang.indexOf(browserLang) !== 0) {
                                     // Prefix does not match

lib/api/api.go

@@ -1483,11 +1483,33 @@ func (*service) getDeviceID(w http.ResponseWriter, r *http.Request) {
 
 func (*service) getLang(w http.ResponseWriter, r *http.Request) {
 	lang := r.Header.Get("Accept-Language")
-	var langs []string
+	var weights = make(map[string]float64)
 	for _, l := range strings.Split(lang, ",") {
 		parts := strings.SplitN(l, ";", 2)
-		langs = append(langs, strings.ToLower(strings.TrimSpace(parts[0])))
+		code := strings.ToLower(strings.TrimSpace(parts[0]))
+		weights[code] = 1.0
+		if len(parts) < 2 {
+			continue
+		}
+		weight := strings.ToLower(strings.TrimSpace(parts[1]))
+		if !strings.HasPrefix(weight, "q=") {
+			continue
+		}
+		if q, err := strconv.ParseFloat(weight[2:], 32); err != nil {
+			// Completely dismiss entries with invalid weight
+			delete(weights, code)
+		} else {
+			weights[code] = q
+		}
 	}
+	var langs = make([]string, 0, len(weights))
+	for code := range weights {
+		langs = append(langs, code)
+	}
+	// Reorder by descending q value
+	sort.SliceStable(langs, func(i, j int) bool {
+		return weights[langs[i]] > weights[langs[j]]
+	})
 	sendJSON(w, langs)
 }