there is no property for this. So it cannot be done.
However, there is a workaround:
You could create a font with flipped character glyphs and assign the font to the text.
See into the IndyFont script by Jongware and Marc Autret that could assist for creating the font:
The other workaround would be:
Use anchored text frames that are flipped.
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Could you please elaborate how to "use anchored text frames that are flipped."
As Uwe said, there doesn't seem to be a property for flipping a text character in a frame. But, closely related to this (just in case it is useful to you), you can ROTATE a character in a text frame by switching to the Japanese Paragraph Composer and using the characterRotation property:
app.selection.composer = "$ID/HL Composer J"; app.selection..characterRotation = 180;
See also https://indesignsecrets.com/free-script-to-rotate-selected-characters.php for a script a wrote that uses this feature.
Thanks for your help...
i've tried but i cannot use rotation for this, it is not the same as flip effect.
Thanks, I didn't know that other composers can not rotate the characters.
Hi Ten A,
actually the other composers can rotate characters.
But not very "orderly". Maybe a feature that wasn't fully developed.
If you want to be sure that everything works as expected you need the Japanese Paragraph Composer.
But that leads us away from eboda_snaf's problem.
Already suggested two solutions:
 Use a new or special font where the glyph shapes are flipped.
 Use anchored text frames that are flipped.
Any other solution is welcome, but currently I cannot see one…
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"…Could you please elaborate how to "use anchored text frames that are flipped." …"
see the screenshots from my German InDesign below:
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