Add language statistics API endpoint (#11737)

* Add language statistics API

* Add tests

routers/api/v1/swagger/repo.go

@@ -302,3 +302,10 @@ type swaggerTopicNames struct {
 	// in: body
 	Body api.TopicName `json:"body"`
 }
+
+// LanguageStatistics
+// swagger:response LanguageStatistics
+type swaggerLanguageStatistics struct {
+	// in: body
+	Body map[string]int64 `json:"body"`
+}