I'd recommend going for bottom-left to top-right in LTR languages and bottom-right to top-left in RLT languages.
Any objections @Mooeypoo or @Pginer-WMF ?

That's actually a really interesting problem. I am actually not entirely sure that cross-out lines are rtl/ltr related, similarly to check-marks.
For example, I am left-handed, and my check-mark is flipped from a person who'd write it right-handed. That's not because of RTL, but because I am left-handed, and the motion of my hand is that way.

I am not sure we need to flip those, but since I am not sure if there is a convention in this case, I would defer to @Amire80's judgment (and ping him on this at the same time)

@Mooeypoo Thanks for the feedback, I'd orientate (as I linked in my comment) on street sign standards. They have definitely included some user tests ;)
Looking forward to further comments by @Amire80

It's more an esthetic issue, if at all - how do they look next to other icons. Consistency may be important here. But they are understandable in any direction.

Thanks for asking!

There's another finding: Android (Material Design) uses in their LTR version top-left to bottom-right cross-out lines to indicate off state. With reading direction left-to-right several icons are also graph like featuring a bottom-left to top-right iconography. One important example is https://design.google.com/icons/#ic_signal_cellular_off

Therefore it seems advantegous to have the cross-out lines top-left to bottom-right.

Change 297186 had a related patch set uploaded (by VolkerE):
Unify cross-out lines direction in icons

https://gerrit.wikimedia.org/r/297186

Change 297186 merged by jenkins-bot:
icons: Unify cross-out lines direction to top-left/bottom-right

https://gerrit.wikimedia.org/r/297186