Proper way to test a specific locale

[Alex Villací­s Lasso]

Could somebody indicate me on the proper way to write a test for one 
specific locale? By this I mean, even if the user has LANG=en defined in 
the environment, the test should (temporarily) force the code path taken 
as if LANG=es_EC was defined in the environment. I am asking this 
because I want to write a test that demonstrates a difference between 
floating-point formatting in native oleaut32 versus builtin, that only 
shows up when a locale other than LANG=en is used, and I want this test 
to actually test the difference even for US/UK testers.

Feel free to ask me for any clarifications.

Thanks for your attention.

Alex Villacís Lasso