It's a couple of years since I was north of Inverness, don't recall the roadsigns at all but then I knew where I was going.
When they first introduced Gaelic roadsigns in the western isles in the 80's, my Stornoway-born, Gaelic first language speaking friend pissed herself laughing. Like most native speakers back then, she was illiterate...
(Fully literate in English, though.)
These days, there are fewer native speakers, but most who learn in school learn to read and write at the same time as they learn to speak. I have no objection to Gaelic roadsigns, they don't cost anything more than a wee bitty paint.