I see the same thing. The messages are there though: https://en.wikipedia.beta.wmflabs.org/wiki/MediaWiki:Pagetriage-del-tags-dbu5-label
I'm at a loss at how some messages work but others don't - caching issues?
Adding this message to ext.pageTriage.defaultDeletionTagsOptions fixes this problem, however I'm not sure if it's the correct solution with all these modules.
By the way Danny - do you need help with installing MediaWiki? It's really hard to review your patches if you haven't tested them yourself.