Yes, you can play MDFC cards as lands from your graveyard with Crucible of Worlds, and from the top of your library with Courser of Kruphix.
Those saying you otherwise may be reading too much into the second's ability wording, arguing they are not "land cards" while still on top before being played; however, the ability's current Oracle text reads: "You may play lands from the top of your library.", so this is not an issue.
To be clear, the intent is that with Crucible of Worlds or Courser of Kruphix, you can play the back face of a modal double-faced card as a land if that face is a land, regardless of what types the front face has. (Thus, for example, the intent is that you can play the back face of Sejiri Shelter, namely Sejiri Glacier, as a land from your graveyard or the top of your library this way, respectively, even though the front face is not a land and that card has only the front face's characteristics in the graveyard or library [C.R. 711.4a].) This is so even though unfortunately, the comprehensive rules at the time of this writing don't clearly address the case of playing modal double-faced cards as lands from a player's graveyard or library. It is hoped (and perhaps even expected) that those rules will change in the future to better reflect this intent.With the rule update for Commander Legends, C.R. 711.8 now covers the case of a player playing a modal double-faced card as a land. That player "chooses one of its faces that's a land before putting it onto the battlefield. It enters the battlefield with that face up." See also this thread.
EDIT (Nov. 20): Edited in view of recent rule update.
Can some1 clarify for me the following.
As I know you can play MDFC with Crucible of Worlds from your graveyard, right ?
But can you also play MDFC from your library with Courser of Kruphix ?
If one case is yes, and the other no, what is the difference ?
Those saying you otherwise may be reading too much into the second's ability wording, arguing they are not "land cards" while still on top before being played; however, the ability's current Oracle text reads: "You may play lands from the top of your library.", so this is not an issue.
This is so even though unfortunately, the comprehensive rules at the time of this writing don't clearly address the case of playing modal double-faced cards as lands from a player's graveyard or library. It is hoped (and perhaps even expected) that those rules will change in the future to better reflect this intent.With the rule update for Commander Legends, C.R. 711.8 now covers the case of a player playing a modal double-faced card as a land. That player "chooses one of its faces that's a land before putting it onto the battlefield. It enters the battlefield with that face up." See also this thread.EDIT (Nov. 20): Edited in view of recent rule update.