From what percentage is a foreign language approximately usable? Are those missing percentages disturbing to the end users?
There is no acceptable percentage because it depends on which yaml files have been translated. A translation is acceptable for the users if at least the
client.yaml file has been translated almost completely.
Okay, so that would be a better indicator for usability than the percentage at Discourse localization ?
How is the Dutch language? Is there a good Dutch introduction and instruction available for new users?
Dutch language is in a good shape, from what I see there are only few untranslated strings. The translation of some plugins is missing, perhaps the translation of the narrative plugin (discobot) should be pushed but for the rest it is certainly usable.