If i set my MAC Satin Finish foundation with a mattifying translucent powder, will it lose its sheen?

not unless you wash it off first.

If you want to set your foundation so it doesn't lose that luminous finish the MAC product gives it then avoid any powders that are matte, this would include some of the thicker mineral powders. Stila makes a really fine translucent powder that should compliment the foundation. A second alternative would be to skip the translucent powder and use a finishing spray (Urban Decay makes a nice one) and spray it directly over your foundation. The spray will add a little extra dewiness to your face too! If you do stick with a translucent powder use a large fluffy makeup brush, dab it in the powder and then blow lightly on the brush so the powder isn't too concentrated in one area, then sweep evenly over your face.

Stay Matte by Rimmel London, or Dream Matte by Maybelline NewYork (not sure if they have it in translucent, though); both are super popular and effective pressed powders, designed for keeping face 'matte' If you like loose powders, Maybelline's Mineral Power Translucent Powder in Finish Veil. (I think it smells kinda funny, like minerals, and application is more messy.)

I use sheen-y makeup too, it is really nice. I think if the powder is made for setting foundation it will not give any coverage therefore your skin won't loose that shimmer. If you set it with pressed powder makeup that offers some coverage, yes that would give your skin a more matte look.