tools: align locale diagnostics and generate native language names

tools/cli/upstream.go

@@ -20,11 +20,7 @@ func UpstreamCommand() *cli.Command {
 				return err
 			}
 			logging.Info("Upstream", "%s: %d mail files, %d languages", s.Lock.Tag, len(s.Templates), len(s.Locales))
-			for _, lang := range s.Languages() {
-				if keys := missing[lang]; len(keys) > 0 {
-					logging.Warn("Locale", "%s -> en-US: %v", lang, keys)
-				}
-			}
+			logging.WarnMissingTranslations(missing)
 			return nil
 		}},
 	}}