Textastic fully supports Chinese text encodings, but the automatic character encoding detection may sometimes choose the wrong encoding.
In this case, please try to tap and hold a file you want to open and choose "Open with Encoding". Then select the correct Chinese encoding and the file should display correctly.
You can see a screenshot of this menu in this topic of the manual: https://www.textasticapp.com/v6/manual/lessons/How_can_I_view_an_image_file_or_open_a_file_in_another_app.html
Customer support service by UserEcho