Well there are numerous differences besides the cost. What is your main use? Simple titles, or complex 3D animations? 3D Production Studio is capable of much more than 3D titles, where 3D 3.5 does 3D titles, as good as Production Studio, but not the more complex animations.
Downside of Production Studio, your projects take forever to render. If you have a lot of patience, go with Production Studio. IF not well maybe 3.5.
Multilanguage, I really don't know. It handles english, and that's all I needed...
You're probably aware they are both EOL (End of Life) products. I'm really surprised that Corel is still selling them. They no longer support either, and there will be no more patches than what's already available. Both are not
Vista certified or compatible.