https://www.w3.org/International/questions/qa-ltr-scripts-in-rtl#styling_mirroring

If the font automatically reverses the images of the letters, that should take care of any necessary mirroring. However, the CSS transformation hack probably won't help, because it is only intended for use with short inline phrases. That said, it is perhaps less likely anyway that reversed letters are needed for scenarios that use the styled approach.

What needs to be mirrored is not necessarily letters, but also numbers, punctuation, word separators, etc., so I think "characters" or "character glyphs" is more accurate than "letters" here.