Slow play

[Zappa]

If someone is playing with their whole deck that is foil, and every single card is in a foreign language. You're trying to grasp what a lot the card does, its effect and abilities and so on, and how it will affect board positioning and what those cards will do for him. Can you really be accused of "slow play"? Specially when you're trying to save your one and only counter for something that matters the most.

[Nelis]

Good question because I have bad memory for remembering what cards exactly do.

[dorsch]

This is the official definiton of slow play:


Definition
Players who take longer than is reasonably required to complete game actions are engaging in Slow Play. If a judge believes a player is intentionally playing slowly to take advantage of a time limit, the infraction is Cheating — Stalling.
Examples
A. A player repeatedly reviews his opponent’s graveyard without any significant change in game state.
B. A player spends time writing down the contents of an opponent's deck while resolving Thought Hemorrhage.
C. After 3 minutes into a round at a Pro Tour™ Qualifier, a player has not completed his shuffling.
D. A player gets up from his seat to look at standings, or goes to the bathroom without permission of an official.

Wether you receive a penalty for slow play depends A LOT on the exact circumstances. I don't believe any judge will give you a penalty, just because you want to get the meaning of your opponent's cards. You should even ask the judge for the official oracle text of cards you don't know.
BUT obviously the judge might consider you play slowly, if you need way too much time to think about board positions, even if that is because of the unfamiliar cards.