I agree completely with Lughaidh on both counts.. Yes, 'Francach' etc can be adjective or noun, but while you can say 'Tá an carr/bia sin Francach' [adjective], with people you have to use the copula: 'Is Francach an fear sin' [noun], or 'Is duine/fear Francach é [adjective].
I think it's the same in French and Spanish: 'je suis francais/soy espanol', where francais and espanol may appear to be adjectives but are really nouns. It's just that the indefinite article is omitted in sentences of that type: 'soy estudiante/je suis professeur etc.

As for the direct relative clause '...who isn't French', while 'nach Francach' is perfectly fine, these are often turned unnecessarily into indirect relatives: 'nach Francach é'. However, in the case of an affirmative direct relative such as 'the only one in my family who is French', I think it's always converted to indirect: 'ar Francach é' rather than the direct 'is Francach'. I don't know why.

I will have to learn the terminology such as 'direct relative clause' etc. I am more familiar with my few terms in Irish, so forgive my lack of ability on definitions, etc.

Anyway, the way I would put
The only one in my family who is French is xx
would be
An t-aon Fhrancach atá sa teaghlach ná xx

xx is the only one in my family who is French
Is é xx an t-aon Fhrancach atá sa teaghlach

In both cases the 'my' would be understood

_________________
Bí cinnte de go nglacfaidh triúr le gach aistriúchán a thabharfar.
Be sure to get three in agreement with a translation given.

Any thoughts on ....ar Francach é vs ...nach Francach (with no é) (in terms of structure/validity)? Errigal and Lughaidh say the second one is okay, so it probably is - it is just so alien to anything I've ever heard that I must ask.

BTW, surely, a Bhraoin, you know how important an uppercase letter is in a word such as Francach

Sorry about that... I'll fix the post.
Thanks for that!!

_________________
Bí cinnte de go nglacfaidh triúr le gach aistriúchán a thabharfar.
Be sure to get three in agreement with a translation given.

a Bhraoin - what I wanted to say is the other way round - not that I'm the only Francach in the family but that all the others are and I'm not - de mo mhuintir (because they're originally from Auvergne, Limousin srl and now partly from Corsica and the older generation speak the local languages)