Restraining myself from looking up a reference book, I will give others a chance.

The only opera I have heard of by Montemezzi is L'amore di tre re, which I thought was set in some misty time far away.  But I may be wrong...

OK, guv, it's a fair cop.  You win.

I've looked up the reference books, and I can't find a thing.  In fact I can't find Moncayo at all in the Viking Opera Guide.  The only composer with a name beginning Monc is Lionel Monckton, so I assume he is very recent.  Nor in Kobbe.

And Montemezzi seems a one work guy.

But I did notice on the shelves Gervase Hughes Composers of Operettas, in which I learn that Jolly Roger (1932) together with The Pride of the Regiment (1932) were composed by Walter Leigh to books by V Clinton Baddeley.  I remember it on Radio 3 as great fun.

Meanwhile Darius Milhaud's utterly forgotten opera CHRISTOPHE COLOMB is a quasi-autobiographical account from the old dying explorer, relating the tales of his many adventures.  I would imagine these feature S America in some considerable extent, but I've never heard this piece or seen a score, so I can't say for sure.

There's a Spontini opera called FERNANDO CORTEZ, and telling the life of the conquistador - but again, I don't know a note of this stuff.  Spontini is a rather talented cove, so there might be merit in excavating the piece?

The oldest opera written in Latin America (although by Spanish emigres) is Torrejón y Velasco's LA PURPURA DE LA ROSA, commissioned by the Viceroy of Peru in 1659,  with music by Juan Hidalgo,  to celebrate the marriage of the French King Louis with the infanta Maria Theresa.  The music survives, and I believe Andrew Lawrence-King and The Harp Consort have plans to stage the thing next year, since it comes up for an Anniversary of sorts

Would anyone like to mention the Zarzuela tradition that took root in Latin America from its native Spain?

Well,

Some obvious contenders:

Tannhauser - the shepherd whose piping is heard after Tannhauser legs it from the eternal tedium of the Venusberg;
Tristan und Isolde - the shepherd's pipe in Act 3
Il Ritorno d'Ulisse - Act 2 begins with the shepherd Eumetes singing of his contented lot
Evgeny Onegin - As dawn breaks after Tatiana has written her letter to Onegin, we hear a shepherd's pipe outside

Less obvious: D'Albert's Tiefland, an underrated work, is all about shepherds, as is Birtwistle's Yan Tan Tethera, where the piping of a shepherd gone bad, luring sheep into the mountain IIRC, is a key part of the action.

I don't think The Pilgrim's Progress counts.  The Shepherds of the Delectable Mountains are not overheard, but at the centre of the action.

The most obvious one to me is the shepherd boy at the start of Act III of Tosca, who is heard off-stage.

Shepherds appear in Enescu's Oedipe, proclaiming his birth.

In the opening scene of Monteverdi's ORFEO, the shepherd swain are singing and dancing in anticipation of Orfeo's forthcoming marriage to Eurydice.

BASTIEN ET BASTIENNE are shepherd and shepherdess respectively (and I play my Joker for double points by claiming the same characters in Rousseau's opera LE DEVIN DU VILLAGE).

Lel' is the shepherd with whom SNOW MAIDEN falls in love, and he has some dances accompanied by his shepherd's pipe (a clarinet, in the score).

Since we're unlikely to have a separate round on goatherds (as distinct from shepherds) there's one in DINORAH, although I don't think Dinorah herself counts as her goat is a pet and not a farm animal.