Need help convincing my Mexican wife

Thank you
Let it be her idea.
If you tell her what you want, she will reject it. It is not her want, it is yours.

You can make comments about her that can help her open up her honesty. Such as if she might think some other guy is good looking; you can reassure her with, "Just because you are not single does not make you blind." When confronted with something undeniable, it is easier to accept a truth you may not want to accept otherwise.
As she gets more comfortable with open honesty, it will become much easier to give open honest.
If she is open to the idea, it will begin to work out in Her Own Mind. Keeping that door open so she can express it to you will make it easier for her to overcome her own objections.

But remember... Some people just aint in it, no matter what. She may be or may not be. So if she isn't at the end of the day, she Just Isn't. Respect that as you wish to be respected. It's a Fantasy of yours. Not the end of the world if it is one of many many many fantasies that do not come true.
Accepting this part can actually help you BOTH to relax and be more open and direct in time. Currently.... you are probably hot and tense on this topic...