Yes, you can play the land face of the modal double-faced card (MDFC) from the top of your library here. This may seem a bit counter-intuitive, as the modal double-faced card is not considered a land card while in your library and it only has the characteristics of its front face (spell face), but just like when it's in your hand, the card has the built-in capacity of being played as a land, if you can currently play a land from where it is. Radha doesn't care that it's not a land card before you take the action of playing it as a land, she allows you to take the action of playing a land from your library, and that's what you're doing.
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
The intent is that a modal double-faced card can be played as a land from the library with Radha if either of its faces is a land.But unfortunately, as far as I can tell, nothing in the comprehensive rules clearly addresses the case of playing a modal double-faced card as a land from a player's library (or other zones than the stack or battlefield), as opposed to the case of casting a modal double-faced spell from such zones (e.g., C.R. 711.7). This is so even though the intent is arguably that a player can choose which face of that card to play this way just like the case when a player casts a modal double-faced spell (see, for example, the rulings for Clearwater Pathway).
In this respect, all the rules say on this matter at the time of this writing is that a double-faced card (modal or otherwise) found outside the stack or the battlefield "has only the characteristics of its front face" (C.R. 711.4a), and that if a modal double-faced card whose "front face isn't a permanent card" would enter the battlefield because a player is instructed to put it there, the card stays in its current zone (C.R. 711.9b). (Note also that C.R. 601.3 applies only to casting spells, not playing lands.)
With the rule update for Commander Legends, C.R. 712.8 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."
EDIT: Edited after comment 6 was posted.
EDIT (Nov. 20): Edited in view of recent rule update.
EDIT (Mar. 2): Correct rule citation.
EDIT (Mar. 30): Strike out more text for correctness.
EDIT (Sep. 21, 2021): One rule was renumbered in the meantime.
Unfortunately, as far as I can tell, nothing in the comprehensive rules clearly addresses the case of playing a modal double-faced card as a land from a player's library (or other zones than the stack or battlefield), as opposed to the case of casting a modal double-faced spell from such zones (e.g., C.R. 711.7). This is so even though the intent is arguably that a player can choose which face of that card to play this way just like the case when a player casts a modal double-faced spell (see, for example, the rulings for Clearwater Pathway).
In this respect, all the rules say at the time of this writing is that a double-faced card (modal or otherwise) found outside the stack or the battlefield "has only the characteristics of its front face" (C.R. 711.4a), and that if a modal double-faced card whose "front face isn't a permanent card" would enter the battlefield because a player is instructed to put it there, the card stays in its current zone (C.R. 711.9b). (Note also that C.R. 601.3 applies only to casting spells, not playing lands.)
The way you seem to read the rules is that a Modal Double Faced Card doesn't have a back face at all. At least, not in a way that is relevant. Based on the rules of course.
That is obviously nonsense and I am pretty sure that you are not suggesting it can't be played as a land at all. But it would probably be best for people like OP that are asking the question if you clarified it to be "the rules don't technically support it, but...". Unless you *are* suggesting the answer MadMage provided is wrong...
In any case, I wanted to provide something supporting the answer MadMage gave so OP doesn't walk away with any doubts about the validity of the answer or the ability to do what they are suggesting:
The way you seem to read the rules is that a Modal Double Faced Card doesn't have a back face at all. At least, not in a way that is relevant. Based on the rules of course.
That is obviously nonsense and I am pretty sure that you are not suggesting it can't be played as a land at all. But it would probably be best for people like OP that are asking the question if you clarified it to be "the rules don't technically support it, but...".
Modal double-faced cards have a back face just like other double-faced cards (C.R. 712.1d). However, at the time of this writing, under the comprehensive rules, a modal double-faced card can be played as a land only if its front face is a land, and then only its front face can be played this way even if the back face is also a land, but that is arguably not the intent. Rather, the intent is arguably that if a modal double-faced card is on the top of the appropriate player's library (regardless of what types its front face has), that card's back face can nevertheless be played as a land with Radha if that face is a land. It is hoped that the comprehensive rules will change to better reflect this intent. And in the meantime, in casual unsanctioned games, the players in the game can agree on modifications to the comprehensive rules ("house rules") to better reflect the intent of those rules.
With the rule update for Commander Legends, C.R. 712.8 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."
EDIT (Nov. 20): Edited in view of recent rule update.
EDIT (Jun. 19, 2021): Correct rule citation.
EDIT (Sep. 21, 2021): Some rules were renumbered in the meantime.
The Zendikar Rising Release Notes clearly confirm that a MDFC land can be played thru Radha, even when its front face is not a land:
To determine whether it is legal to play a modal double-faced card, consider only the characteristics of the face you're playing and ignore the other face's characteristics.
and
If an effect allows you to play a land or cast a spell from among a group of cards, you may play or cast a modal double-faced card with any face that fits the criteria of that effect. For example, if Sejiri Shelter / Sejiri Glacier is in your graveyard and an effect allows you to play lands from your graveyard, you could play Sejiri Glacier.
Release Notes are not an official part of the rules, but this clearly describes how MDFC are supposed to work.
The Comprehensive Rules obviously need reworking in order to actually reflect this.
But unfortunately, as far as I can tell, nothing in the comprehensive rules clearly addresses the case of playing a modal double-faced card as a land from a player's library (or other zones than the stack or battlefield), as opposed to the case of casting a modal double-faced spell from such zones (e.g., C.R. 711.7). This is so even though the intent is arguably that a player can choose which face of that card to play this way just like the case when a player casts a modal double-faced spell (see, for example, the rulings for Clearwater Pathway).In this respect, all the rules say on this matter at the time of this writing is that a double-faced card (modal or otherwise) found outside the stack or the battlefield "has only the characteristics of its front face" (C.R. 711.4a), and that if a modal double-faced card whose "front face isn't a permanent card" would enter the battlefield because a player is instructed to put it there, the card stays in its current zone (C.R. 711.9b).(Note also that C.R. 601.3 applies only to casting spells, not playing lands.)With the rule update for Commander Legends, C.R. 712.8 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."
EDIT: Edited after comment 6 was posted.
EDIT (Nov. 20): Edited in view of recent rule update.
EDIT (Mar. 2): Correct rule citation.
EDIT (Mar. 30): Strike out more text for correctness.
EDIT (Sep. 21, 2021): One rule was renumbered in the meantime.
That is obviously nonsense and I am pretty sure that you are not suggesting it can't be played as a land at all. But it would probably be best for people like OP that are asking the question if you clarified it to be "the rules don't technically support it, but...". Unless you *are* suggesting the answer MadMage provided is wrong...
In fact, Tabak already knows of this oversight:
https://twitter.com/WotC_Matt/status/1310663566119239682
In any case, I wanted to provide something supporting the answer MadMage gave so OP doesn't walk away with any doubts about the validity of the answer or the ability to do what they are suggesting:
https://twitter.com/WotC_Matt/status/1306399857670725632
However, at the time of this writing, under the comprehensive rules, a modal double-faced card can be played as a land only if its front face is a land, and then only its front face can be played this way even if the back face is also a land, but that is arguably not the intent. Rather, the intent is arguably thatif a modal double-faced card is on the top of the appropriate player's library (regardless of what types its front face has), that card's back face can nevertheless be played as a land with Radha if that face is a land.It is hoped that the comprehensive rules will change to better reflect this intent. And in the meantime, in casual unsanctioned games, the players in the game can agree on modifications to the comprehensive rules ("house rules") to better reflect the intent of those rules.With the rule update for Commander Legends, C.R. 712.8 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."
EDIT (Nov. 20): Edited in view of recent rule update.
EDIT (Jun. 19, 2021): Correct rule citation.
EDIT (Sep. 21, 2021): Some rules were renumbered in the meantime.
and
Release Notes are not an official part of the rules, but this clearly describes how MDFC are supposed to work.
The Comprehensive Rules obviously need reworking in order to actually reflect this.
https://magic.wizards.com/en/articles/archive/feature/zendikar-rising-release-notes-2020-09-10
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules