How to make the Charts module use the Highcharts legend option "useHTML" (required for RTL)?
You should be able to get this to work using the approach as described in the issue about Pass library-specific options via existing #raw_options, AFTER you apply the patch in comment nr 4 of this issue.

Disclosure: I'm a (co-)maintainer of this module.

PS 1: Would be nice/great if you could confirm that this patch also worked for you.

PS 2: after a similar patch becomes available to Google charts also, both patches will be committed.