Add language statistics API endpoint (#11737)

* Add language statistics API

* Add tests

routers/api/v1/api.go

@@ -855,6 +855,7 @@ func RegisterRoutes(m *macaron.Macaron) {
 							Delete(reqToken(), repo.DeleteTopic)
 					}, reqAdmin())
 				}, reqAnyRepoReader())
+				m.Get("/languages", reqRepoReader(models.UnitTypeCode), repo.GetLanguages)
 			}, repoAssignment())
 		})