rl. However, this will save the file with the characters in the order you typed them in. If you want to have it save it reversed, type
:%!rev before saving.
If you use the
ri option, the inserting is done backwards. You could probably map this to a key combination, but that's up to you.
Here is the appropriate section of vim help:
o Typing backwards *ins-reverse*
In lieu of using full-fledged the 'rightleft' option, one can opt for
reverse insertion. When the 'revins' (reverse insert) option is set,
inserting happens backwards. This can be used to type right-to-left
text. When inserting characters the cursor is not moved and the text
moves rightwards. A <BS> deletes the character under the cursor.
CTRL-W and CTRL-U also work in the opposite direction. <BS>, CTRL-W
and CTRL-U do not stop at the start of insert or end of line, no matter
how the 'backspace' option is set.
There is no reverse replace mode (yet).
If the 'showmode' option is set, "-- REVERSE INSERT --" will be shown
in the status line when reverse Insert mode is active.