The "force RTL" setting in the developer options only makes the locale a right-to-left locale if it wasn't already. That is, if your native language isn't right-to-left, this option makes it draw right-to-left anyway. It allows developers who can't read any right-to-left languages to test that their apps work in a right-to-left language.
The option can't make apps that don't support RTL support it. The app author needs to update their app to support RTL, and obviously they're only going to do this if they're going to offer translations to RTL languages. An app that only supports English probably won't be written to support RTL.
As a rule, it's a bad idea to start changing developer settings without understanding what they do. They're hidden behind this screen precisely because they can cause unexpected or undesirable behaviour.