The Code Blocks ( insert->code blocks) section only supports the text highlighting(I guess its GeSHi). The autocompletion and snippets of the language inputted are not enabled. May be the call to the CodeEditor (Ace)are not done properly or the languages and respective mode are not set properly.

The CodeEditor seperately is working fine ,if we take the case of the insert math formula where the autocompletion is working fine.
The math formula , although uses an instance defined by the visual editor ( modules->ve-mw->ui->widgets->ve.ui.MWAceEditor.js) .

It was not clear to me where the codeblock calls the instance of the ace editor are made.

Feedback discussion :
https://www.mediawiki.org/wiki/Topic:T5x13al94xj3r6sa