rather than converting directly, we have `lcode_to_*` functions in `shared/i18n`.  Either we have one there to use (I think we do...), or you should create one and call that on `lang_code`.

In addition, `request.language` is under our control.  Internally, we should be using the IETF code.  So I suggest, in the end:
* Make sure `request.language` is using the IETF code.  If not, change it; then search the codebase for all references to `request.language`, and make sure anything being compared is converted via `lcode_to_ietf`.
* Convert `lang_code` to IETF version here.