For sites that have enabled the support mixed text direction setting, the text direction of posts will be set based on the language used in the post. See What does the support mixed text direction setting do? for more details. This setting is not enabled on try.discourse.org.
Your screenshot brings up a good point though. the FAQ, Terms of Service, and Privacy pages are not regular Discourse posts, so they do not get their direction set correctly even if the support mixed text direction setting is enabled.