If you don't have high speed internationally, it will be slow. Possibly too slow to handle.
Also, you're under the rules of who you're roaming on. If the roaming partner doesn't allow hotspot, T-Mo can't do anything.
"Tethering/Smartphone Mobile Hotspot
Even though the included data roaming service that comes with Simple Choice allows you to use data for your phone, it doesn't allow the ability to share with other devices by tethering through Smartphone Mobile Hotspot. If you need to share data through tethering, then we suggest adding an international data pass. Not only does it allow your device to be used as a Smartphone Mobile Hotspot, but it gives you speeds of up to 3G, depending on what country you're visiting."
T-Mobile's Hotspot app should not allow free tethering but a non-T-Mobile solution might be able to circumvent that. I wouldn't know.