Currently there isn't an easy way to specify the FontFamily of the RadLegend items, I confirm this is a valid feature request, so I've updated its status to "Unplanned". We will keep an eye on the item and will schedule it according to the demand.
In the meantime, from the screenshot it seems you've come across the following bug report related to RadLegend on iOS:
I've updated the priority of the issue per your report. I am currently researching for a suitable workaround through a renderer on iOS, I will write back with more details later today.
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