It's not yet possible in the current version, sorry.

Thank a lot for letting me know!

Hello,

currently, only syntax highlighting is supported for Go.

It's possible to add custom code completion files. You can find the existing code completion files for HTML, CSS, JavaScript, C, Objective-C and PHP at https://github.com/blach/Textastic-Customization/tree/master/CodeCompletion

I'm glad that it works for you!

This should also work in iOS 13 if you installed a monospaced font using an app like AnyFont.

On iPad you can hide these using the "Additional Keys" setting in Textastic. 

I can reproduce the problem with Japanese text. I'll try to find out what is going on.

I could reproduce and find a workaround for this problem just yesterday. It will be fixed in the next update.

This seems to be a JSX syntax definition for TextMate 2 which injects itself into the built-in JavaScript syntax.

Textastic only supports TextMate 1 (and Sublime Text 2) syntax definitions, which do not support injection.