If you collect a card that is expected to exist in following sets, you might be able to remove some versions from your wants list. These cards don't exist:
|Limited Edition Alpha||
It is said Wizards of the Coast wasn't content with the art received for Volcanic Island and designed another card based on it: Birds of Paradise.
According to Jesper Myrfors, who was art director back then, there was no art available for CoP: Black so that he had to create one himself in no time. It was the only digital art for a Magic card in these days.
|Portuguese 4th Edition BB and WB||
||All missing Portuguese cards are ante cards. Portuguese Magic was mainly introduced for Brazil, where gambling is forbidden.|
|Portuguese Homelands||Timmerian Fiends|
|Portuguese Ice Age||Amulet of Quoz|
|Traditional Chinese and Korean 4th Edition||
||The Chinese censorship removed some cards showing quasi unclothed creatures or religious motives.|
Simplified Chinese 5th Edition
99 cards were taken out for different reasons:
Missing Chinese Foils
Some sets were printed in Chinese but had no foil versions. Boosters of these contained English foil cards instead:
— Traditional Chinese Urza's Legacy
— Traditional Chinese Urza's Destiny
— Simplified Chinese Invasion
Wizards had planned to release Coldsnap in all active languages. But theme decks were cut in Chinese, and Russian doesn't exist at all. Hence, it was the only standard legal booster set not supporting all of the then active languages.
Misleading Data in the Web
Several non-existing localized versions of sets are listed to exist in the official Wizards of the Coast data base Gatherer, and older existing ones are not. This erroneous data has been more or less used for all multilingual online stores and community made MtG card search sites.
The correct info can be found in the Card Set Archive of Wizards of the Coast (note: not all types of product are available in the listed languages). While that page is still incomplete in terms of omitting some special and foreign-only sets as well as any promo and reward programs, it doesn't contain any blatant misinformation.
Also be cautious about circulating spread sheets which aim to list all sets and languages they were printed in. I haven't come along one that's both complete and free of errors.