Code completion for EditorTextField wrapped in DialogWrapper

Hi,
I have a plugin with EditorTextField embedded in DialogWrapper. I get text from XML file specific tags, so editor's text has no original file. I created custom language and pass it as a parameter to EditorTextField constructor and syntax highlighting works fine, but I can't make code completion work. I know I can use CompletionContributor. 
I created and registered my CompletionContributor in plugin.xml, but it doesn't work. I suspect I need to use existing file, not a code snippet. Is it possible to use it in my case?