What exactly does not work?

You can add your own TextMate-compatible syntax definitions to Textastic. Have a look at the manual: http://www.textasticapp.com/v4/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html

You can find a Flex TextMate bundle with an .mxml language definition at https://github.com/simongregory/flex-tmbundle

Sorry, but Apple doesn't allow compilers or interpreters in apps that also allow file downloads and file sharing.

Sorry, but I don't speak Russian. Can you please post in English (or German)?

It would sure be nice, but I think it would be out of the scope of Textastic. But it sounds like a good idea for a new app.

The cursor movement happens instantly, but the the scrolling happens after a very short delay. This is not only when using external keyboard, but also when you move the cursor with your finger.

I'll consider improving the behavior.

Just tap on the blue "Search" button to dismiss the keyboard. Then you can tap on the "Done" button.

I guess that would count as sharing, sorry. Also, it's not possible for an app to access files in the sandbox of another app. 

No, I added the same feature to the iPad version. It's just not in the iPad manual yet, that's why I pointed you to the iPhone manual.

Make sure you have the latest version (4.2.1) installed.

Just swipe quickly (!) to the left or right side. This will move the cursor one character at a time. If you use two fingers, the cursor will move a word. If you use three fingers, the cursor will jump to the beginning or end of the line.