Currently, the CodeMirror extension only highlights the <languages /> and <translate> tags, but not the new <tvar name=n>...</tvar> tag syntax.
Support for highlighting the <translate> tag was added in T172456: Syntax highlighting - translate and tvar tags, and the new <tvar> syntax was introduced to Extension:Translate in T274881: Change translation variable (tvar) syntax, but CodeMirror still shows them unstyled.
Note that <tvar> tags are only valid inside the <translate> tags.