Which icons should be mirrored for RTL?
Icons that speak or reflect direction should be mirrored.
In order to trigger the same feeling, or intent to the user across LTR and RTL interfaces, icons with direction associated with them should be mirrored in-order to send the same message, where otherwise it could send the opposite intent.
What is UI mirroring?
The goal of the Windows UI mirroring API is to allow GUI developers to write UI code without having to worry about mirroring the UI for right-to-left locales. The UI is created using well known Windows GUI constructs (HWNDs, Device Contexts, Dialog Boxes, etc.)
What is RTL language support?
Some languages of the world (Arabic, Hebrew etc.) are RTL, meaning they are read right-to-left, instead of left-to-right. Typically in web applications supporting one of these languages, everything is reversed, meaning scroll bars, progress indicators, buttons etc.