[MM-40560] Add Server locale to link preview request (#19156)

* Add Server locale to link preview request

* Update unit tests for post embed

app/post_metadata.go

@@ -549,6 +549,7 @@ func (a *App) getLinkMetadata(requestURL string, timestamp int64, isNewPost bool
 		} else {
 			request.Header.Add("Accept", "image/*")
 			request.Header.Add("Accept", "text/html;q=0.8")
+			request.Header.Add("Accept-Language", *a.Config().LocalizationSettings.DefaultServerLocale)
 
 			client := a.HTTPService().MakeClient(false)
 			client.Timeout = time.Duration(*a.Config().ExperimentalSettings.LinkMetadataTimeoutMilliseconds) * time.Millisecond