I doubt its wayland related. Do you mean the little close button *inside* the menubar ?

Created attachment 1217630 [details]
seems ok for me

This is what I get with 
export LANG=ar_EG.UTF-8
with libreoffice-langpack-ar installed

No, I'm referring to the close button on the title bar. Notice that I *don't* have any RTL langpack installed for libreoffice, so it uses English by itself and the little close button appears on the right, and I'm OK with it. However, the close button on the title bar should appear on the left, like any other application.

Note: it works as I expect under Xorg, so it is somehow related to Wayland.

Hmm, it must be the gtk_widget_set_default_direction we use to make gtk match text direction of the UI with the text direction of the UI language in use. This must additionally affect the toplevel window-manager-a-like settings under wayland. Not too sure what to do about that, seeing as if a user does select to use a LibreOffice RTL UI inside a LTR environment we do want to force gtks direction. *perhaps* we can wiggle things around to try and leave the toplevel widget as the system direction or something of that nature. Probably all fragile stuff though.

That's correct. I tried running gnome-calculator with LANG=en_US.UTF-8 in an RTL locale, and the close button for it also appeared on the right. So, the close button placement follows application's locale rather than desktop locale... :(

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Tried a few things. Beginning to look like that if we continue to change LANGUAGE etc to the user's ui language then its going to affect the built-in csd titlebar. So one solution is to set our own explicit titlebar of the desired direction

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Thanks a lot, it is fixed :)