Currently that text exists outside the WP localisation pipeline, which is why you’re not seeing them appear in the WPML administration panel. You will see other text from the WP Discourse plugin in the WPML “String Translation” interface, but not the text in that screenshot.
This is a bit of a gap in the WP Discourse plugin and I’ve moved it up the priority list. I’ve mostly completed a PR to allow those strings to be translated within WPML (the most widely used WP Multilingual Plugin). We may add support for other multilingual plugins if possible (not all have the same customisability as WPML).
I’ll give you an update on this one early next week.