Alright here is the deal. Tongue of the Sun and Moon is a Secret Language, learn-able and reproduce-able. Also there are many other ways to communicate with all things living (and non living!) using only a single or two languages and without being a monk. Here is how:
Logical Rule #1
Rule on Languages: "By virtue of your race [and it's learned/developed properties], your character can speak, read, and write certain languages."
Logical Rule #2
"Your background might give you access to one or more additional languages of your choice."
Logical Rule #3
"Some of these languages are actually families of languages with many dialects... Creatures that speak different dialects of the same language can communicate with one another.
Logical Rule #4
"With your DM's permission, you can instead choose a language from the Exotic Languages table or a secret language..."
Assertion 1: English definition of Tongue means "spoken language", making Tongue of Son and Moon a Language.
Assertion 2: secret, is anything that is intentionally hidden, or not commonly accessible, ie druidic (ritual/spoken), thieves cant (visual/subliminal dialect) OR in this case Tongue of Sun and Moon.
Assertion 3: "Ki is ONE element of the magic that suffuses all living beings--specifically the element that flows through living bodies."
Assertion 4: Ki and ki points according to the languages definition are not needed to use Tongue of Sun and Moon, therefore nullifying the argument that it is monk only ability. However Tongue of Sun and Moon does require you to "touch the Ki of other minds", in order to "understand all spoken languages" even though you do not need Ki necessarily to do so.
Assertion 5: Tongue of the Sun and Moon is not the only way to Access the "language of the mind", just as becoming a druid or being taught by a druid is not the only way to learn druidic.
Using logical deductive reasoning we see that the language of the mind is the master language from which all languages of living creatures, and their dialects, cants etc. were derived. The language of the mind is a LANGUAGE that is undiscovered, and unnamed and unknowingly accessed by the Language of Sun and Moon as well as potentially accessed in many other ways.
SO wrapping up we have discovered several important things:
- The language of the mind is a hidden secret language. And all living creatures with minds have it flowing through them via the element of ki.
- It is in fact so secret that D&D doesn't know that it has created it.
- The language of the mind is not the languages name, thereby preventing us from picking or learning it as our default language directly because we are not SURE it exists or even how to know if it does let alone reproduce it, even though we see that we are accessing it's results through the secret monk language: Tongue of the Sun and Moon.
- Tongue of the Sun and Moon is only one way to touch the ki of other minds.
- Ki is the magical element that flows through the mind of living things.
- Touching the Ki of other living minds is only one way to access this unnamed language of the mind.
- Other elements of magic that suffuse the multiverse flow through non living things, giving access to the language of the mind possible by many other means.
- Behind the language of the mind is the deeper hidden and undiscovered language of the multiverse, how all things (living and not) communicate/speak to each-other on a subatomic or magical level.
- There are known ways to access the core language of the multiverse however there is no knowledge of what it is actually called.
- With DM's permission starting players are allowed to pick secret languages as their background languages, this may include "Tongue of the Sun and Moon" or "Telepathy" as they are a discovered (in the handbook) secret language.
- There are many other known ways to access the undiscovered languages of the living mind, non living mind, or multiverse, and I list many of them below.
I. DMs permission to acquire Tongue of Sun and Moon for starting language [access Living].
II. Monk level 13, acquire Tongue of Sun and Moon passively [access Living].
III. Create a half-everything race Ranger (8th quarter everything technically), choose your favored enemy to be Formian. This grants you the Hive Mind telepathic language that gives access to the minds of all your same race within 60 ft. Now you can Hive Mind (speak telepathically) with all races within 60 ft because you are technically all races [access Living].
IV. Create a Ranger, choose your favored enemy to be Dragon/Pseudodragon. This grants you Telepathy with any and multiple creatures that know a language that are within 60 ft of you [access Living].
V. Find a magic user, become transfigured into a dragon, or swap out your soul with a dragon, you can now speak telepathically with any living creature [access Living].
VI. Start the game as a rock or other inanimate object that. Hope a magic user transfigures you into a living creature. Learn common. Ask the DM if you can make a check to remember what it was like to be a rock. If Succeed. Make an intelligence check to remember if you could communicate with other inanimate objects. If Succeed. Make intelligence check to determine that method. If succeed. Use that new communication method to communicate with things in the same manner while in your new living form [access non Living].
VII. Become a high level magic user, make checks to create magic devices to discover a. all the elements of magic. b. the names of language of the living and non living minds. c. the name of language of the multiverse. Now make checks to create a magic device(s) that allows you to understand and speak with any creature, inanimate object, or the elements of magic, or the multiverse directly. [access all communication]
VIII. Become a high diety worshiper/patron. Build enough favor with your diety. Meet your diety. Ask your diety to grant you access to the secret languages of minds or the multiverse or to show you how to do so. [access all communication]
IX. Become a level 13 monk. Learn tongue of sun and moon. Get a mage to Transfigure a rock into a living being. Touch the mind of the newly alive being and learn to speak the language of inanimate objects in addition to living objects. [access living and non living]
X. Become a high level mage/scholar. Learn all languages. Increase your intelligence really high. Tell the DM you are rolling the remaining years of your characters life to discover the names of the hidden secret languages and reproduce their practical application. Ask the DM what you need to roll/how many times to succeed. If you succeed. DM has to tell you what it is(Make it up) and your character dies being remembered for the discovery. If you fail your character dies and is buried in an unmarked grave for wasted research. [access all communication]
Probably several other ways, those were all I could think of off the top of my head.