well... I think the folder "www" in "server" would clean when running "gulp build". That means the changes I made in the language file in "locales" would all be removed, and then the language file would be reloaded after "transifex-download" finish. so looks like if I skip "transifex-download", "locales" would be empty after "gulp build" finish.
The temporary way I find out now is to reload my language file when every time after I make some other changes and run "gulp build", but that's a little bit inconvenient.