That would be more sensible, but it is not the correct translation for aon which specifies that you have 'one' moustache. It is easy to miss the difference if you are used to French, German, etc, where the same word means both a(n) and one. Note also that the English words are slightly different but related. No Celtic language apart from Breton has an indefinite article. Compare:
|J'ai une moustache||I have a moustache||Tha stais agam|
|J'ai une moustache||I have one moustache||Tha aon stais agam|
This is not the correct Gaelic for 'I am wearing one moustache'. If it actually on your face it would be
Tha aon stais orm Is one 'tache on-me
The agam sentence is is the way you say you literally have one. Maybe you bought it or found it and put it in your pocket. So you could have any number.
It is rather misleading the way they set this question without making that clear. I think they have just put random numbers with random nouns:
I have six oranges
I have two cats
I have one moustache
I have seven arms
I have three husbands
Yes, it may well have been because he was French-speaking. This link shows a number of examples in French where it is translated into English in the singular, and one where it is translated as whiskers.
And in these two quotes from Proust on the same page I would definitely translate as whiskers.
And how about a quote from The Three Musketeers?
But when we read about a cat's moustaches then there is no doubt it means 'whiskers', as they are also described as vibrisses.
French Wikipedia says a cat has 'a moustache', but then goes on to say
que les scientifiques appellent vibrisses 'that scientists call whiskers'
I suspect that it may be used for a 'handlebar moustache', the sort that sticks out on each side like a pair of whiskers. D
Edit 24/6/21: just seen that a Breton dictionary (you have to search for the term moustache yourself) says
MOUSTACHENN f. -où, souv. MOUSTACHOÙ pl.
which means that the Breton word is often found in the plural.
Firstly, there are unfortunately a number of very unreliable dictionaries around, including some that masquerade as new when they are in fact both old and unreliable. So a statement like this is not much use without saying which dictionary. The most reliable free dictionaries are Am Faclair Beag (modern) and Dwelly (1911) both found at faclair.com. Colin Mark (2003) is also very good with lots of good examples, but it is extremely expensive so it does not compete with the free ones. Learngaelic.scot is I think mostly a copy of AFB.
Mark does indeed give both, with approximately equal prominence. AFB gives both, and Dwelly neither. These dictionaries all give various words that begin with r and phrases that mean things like 'mouth-beard'.
DASG, the corpus of Gaelic, gives 42 hits for stais and only 11 for mustais.
I think this is a situation where we have to accept that a word being in a dictionary says nothing about the commonness of a word, so we should stick with what this course or any teacher you have says unless you can say you have actually heard the word in the wild.
There is also a major problem with mustais which may account for the loss of the first syllable. In Gaelic, all words have the stress on the first syllable (except for a few words with built-in prefixes), so people would struggle to pronounce this the English way. They would either mumble the first syllable, or mishear it as mo stais. When I looked in DASG, I actually saw a number of examples of mo stais or m' stais, including some that did not make sense (such as mo stais agam), so this is clearly what is happening. Mustais is being reanalysed as mo stais to fit Gaelic pronunciation.
As an aside for anyone with a general interest in Celtic languages, Breton has a problem with ch. The problem is that it has two different sorts. The sort you would expect in a Celtic language, pronounced as in Gaelic, and the French sort, because they have so many words borrowed from French. So they retain the ch in words of French origin, such as moustachenn from the French moustache, where the ch is pronounced as in m(o)ustache and stais, but they have changed the spelling of the Celtic ch so you can tell the difference, and it is written c'h. This is very common and it is the dead giveaway for recognising Breton text, as it gives Breton a unique look. Thus Breton loc'h = Gaelic loch (meanings similar but not identical).