Discourse only uses the browser’s locale to set the user’s locale if the set locale from accept language header site setting is enabled. When enabled, the locale will be set for anonymous users from their browser’s language header. This setting also sets the locale for users when they first create an account on the site. That locale can be overridden by the user by going to their Preferences / Interface page and selecting the interface language that they want for the site. Once this has been done, the language headers will be ignored - the user’s chosen interface language overrides the browser’s language header.
If you select a RTL locale for a Discourse forum, all content will be displayed with a RTL layout. If you are on a LTR site, it would be best to select a LTR locale on your preferences page. The only exception to this is if the support mixed text direction site setting is enabled. When that setting is enabled, post content will be parsed for its text direction and the appropriate dir attributes will be added to the post content. You can find more details about that setting here: What does the support mixed text direction setting do?.