This is probably not that easy to implement, but it would certainly benefit a lot of users. I'll consider it for future updates.

Yes, another user had the same problem when running the Baidu input Jailbreak utility.

Thank you, Jonathan!

Oh, and for new templates to be picked up by Textastic, you do not need to restart the app. It's sufficient to just add a new file and select the template.

Ah, ok. My answer above was for adding syntax highlighting support for .mxml files. I didn't realize you wanted to add a template.

You need to json-encode the snippet string. So in your case, you need to escape the quote characters and newlines. 

Replace " with \" and newlines with \n

It would something like this:

snippet = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<s:WindowedApplication xmlns:fx=\"http://ns.adobe.com/mxml/2009\" \n                       xmlns:s=\"library://ns.adobe.com/flex/spark\" \n                       xmlns:mx=\"library://ns.adobe.com/flex/m\""

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)?