I18n: Default regional format to Accept-Language header value (#108468)

* I18n: Default regional format to Accept-Language header value

* comment

* rejigger

* rejigger preferences

pkg/api/index.go

@@ -69,8 +69,18 @@ func (hs *HTTPServer) setIndexViewData(c *contextmodel.ReqContext) (*dtos.IndexV
 	// Locale is used for some number and date/time formatting, whereas language is used just for
 	// translating words in the interface
 	acceptLangHeader := c.Req.Header.Get("Accept-Language")
+	acceptLangHeaderFirstValue := ""
+
+	if len(acceptLangHeader) > 0 {
+		parts := strings.Split(acceptLangHeader, ",")
+		acceptLangHeaderFirstValue = parts[0]
+	}
+
 	locale := "en-US" // default to en formatting, but use the accept-lang header or user's preference
-	var regionalFormat string
+	if acceptLangHeaderFirstValue != "" {
+		locale = acceptLangHeaderFirstValue
+	}
+
 	language := "" // frontend will set the default language
 	urlPrefs := getURLPrefs(c)
 
@@ -80,17 +90,27 @@ func (hs *HTTPServer) setIndexViewData(c *contextmodel.ReqContext) (*dtos.IndexV
 		language = prefs.JSONData.Language
 	}
 
-	if len(acceptLangHeader) > 0 {
-		parts := strings.Split(acceptLangHeader, ",")
-		locale = parts[0]
-	}
-
+	var regionalFormat string
 	if hs.Features.IsEnabled(c.Req.Context(), featuremgmt.FlagLocaleFormatPreference) {
-		regionalFormat = "en" // default to "en", not "en-US", matching the regionalFormat code
+		regionalFormat = "en"
+
+		// We default the regional format (locale) to the Accept-Language header rather than the language preference
+		// mainly because we want to avoid defaulting to en-US for most users who have not set a preference, and we
+		// don't have more specific English language preferences yet.
+
+		// Regional format preference order (from most-preferred to least):
+		// 1. URL parameter
+		// 2. regionalFormat User preference
+		// 3. Accept-Language header
+		// 4. Language preference
 		if urlPrefs.RegionalFormat != "" {
 			regionalFormat = urlPrefs.RegionalFormat
 		} else if prefs.JSONData.RegionalFormat != "" {
 			regionalFormat = prefs.JSONData.RegionalFormat
+		} else if acceptLangHeaderFirstValue != "" {
+			regionalFormat = acceptLangHeaderFirstValue
+		} else if language != "" {
+			regionalFormat = language
 		}
 	}