Yep, ProPack and Poser 5 place the .obj in the library folder. This is a bug. If the .obj file is in a library folder the product is not usable in a non-english Poser version.
If you only want english Poser users then please say so, I'm sure the rest of the world doesn't want to force itself on you.

Then don't advertise the product as usable with Poser 4. It is not.

I don't think so.

Merchant's decision. Well, then I have to do a buyers decision.

I do wonder with which justification Renderosity takes the 50% cut. It is the right of a vendor that his product is thoroughly tested. The store tester has to catch such errors and advise the vendor. This is the only way the interests of a vendor are protected. If he knows the faults he can correct them, if he doesn't know them he can only wonder why his stuff doesn't sell...

Please ask Dark_Whisper what he prefers:
a. A failed product which he can improve without ruffle.
b. A public dismemberment of his product in several forums.

You don't do the vendors any good if you shield them from the real world.

You don't do the Renderosity MP any good either. Other commercial Poser sites don't have a quality problem and they gladly take your customers.