If I understand the ISBN correctly, if I as a publisher, created a Spanish translation here in the US, but was also selling it in, say, Brazil, do I need a separate ISBN issued from Brazil where the printing may occur, or do I just use a number I already own and assign it the translated version only? The physical place of printing the book doesn't matter? Can a 'joint' venture with a foreign publisher remedy the issue of being able to sell it in that country?
An ISBN identifies a publication. Every edition of a book must have a different ISBN. For example, the hardcover edition, the trade paperback, the mass market paperback, the ebook, and the audiobook, all have a different ISBN.
A translation, of course, is a different publication. But any edition can be sold worldwide. When I go to a bookstore in France and buy a book that was published (in English) in the US, that book (which was perhaps shipped from the US) will carry the same ISBN as it would if it were sold in the US. The fact that a book is shipped to and sold in another country does not make it a different publication.
For the ISBN, it also does not matter where a book is printed. Many books sold in one country in Europe have been printed in another country in Europe, where printing was cheaper. There may or may not be a note saying where the book was printed, and different print runs of the same book may be printed in different countries, but they still carry the same ISBN if they are the same edition of the book.
In your case, your book will be published wherever your publishing house has its legal address. This may be the US or Brazil or elsewhere. This book, with the ISBN number of the country of your publishing house, will then be sold in the US and in Brazil.
Only if your publishing house has a subsidiary in both the US and Brazil, and each subsidiary publishes its own edition of this book, will those two editions have different ISBNs. And they will have different ISBNs even when the books are absolutely identical except for the address given on the copyright page. They will also have different ISBNs if they are printed by the same pinter.
From what I understand, ISBN is released by your national agency. So if you are a publisher in Brazil, it is released by Brazil, regardless of the language. If you sell and ship your books across the world, they retain your ISBN number. But if you are a large publishing house with different administrative branches in different countries, then each of them might have their own ISBN.