I saw this question first thing in the morning and actually thought I'd asked it myself for a brief second--what are you doing in my mind? It's a situation I've actually been thinking about a fair bit recently, though even still I can only partially answer.

There is a good chance a Gaelic-speaking recruit would understand enough English to get by--certainly we know on the Jacobite site that many of the most prominent Highland supporters, such as Cameron of Lochiel, did speak it. In The Myth of the Jacobite Clans, Murray Pittock says that "there is Gaelic Jacobite poetry aplenty, but apparently no known orders in Gaelic" (p. 35). He goes on in Chapter 2 to discuss the linguistic profile in the highlands in general, but I can't find the exact page reference now to cite. It gives a fairly high bilingualism rate in this period, Gaelic already on the retreat, so there's a fairly good chance that a given highlander would have enough to get by.

As for treatment, that's a little harder to nail down. There was certainly prejudice against Gaelic speakers in this period (one comparison is to "hottentots" in their primative unintelligibility), and there was a general assumption they were all Jacobites. Particularly with Prince Charles' army deliberately naming itself "The Highland Army" and adopting Highland garb, there was a tendency in England to take it at face value. So from that, I find it somewhat implausible your Highlander would be in the Hanoverian army, though not impossible. He might take a descriptive surname to hide his Gaelicness, like "Dow" from "dubh" (black) or Glass from "glas" (grey) and try to conceal any accent he might have. The stereotype of the teuchter was already around, with the prototypical highlander being rather stupid, primative, and dirty. There are some striking parallels to how the Gaels were seen at this time and how other colonial conquests, like peoples in Africa or North America were seen, particularly in the way they were imagined to be fiercely clannish (read: tribal) and warlike, and after the '45, these assumptions were actually played up to the advantage of some, and leading to the creation of the Highland Regiments (conveniently with ex-Jacobites for officers--see McKillop, More Fruitful Than the Soil).

None of this is directly answering your question, because I haven't read about the exact scenario you propose, but I think it does shed some light on it.