Good question. Probably the answer lies in the xgettext tools/layer. But there is a Perl-based approach that worked for me with similar concerns. A way to do a warning is to do dummy translations of anything in the locale/translation file that isn't already done and fill them up with characters like ▩▩▩▩▩ ▩▩ ▩▩▩▩. It's not a code-based warning or error but becomes obvious in even casual viewing of the site/end-material of what got missed.

No code I can share right now, sorry, and it's not trivial because of the way the tools work. So, this is actually probably terrible advice even though it did work for me… :|