[RPG] Are a Wizard’s “spells known” what they have in their spellbook or what they can prepare

No, you can't write the spells you know as a Bard into your spellbook.

There are 2 ways (outside of levelling as a Wizard) that a Wizard can add spells to their spellbook. The first is by finding them:

When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.

What does it mean to "find" a spell? Some examples are given immediately preceding this:

You might find other spells during your adventures. You could discover a spell recorded on a scroll in an evil wizard’s chest, for example, or in a dusty tome in an ancient library.

While this is not meant to be an exhaustive list, it's strongly suggestive that finding a spell means finding it written down. It certainly doesn't suggest that knowing it is the same as finding it.

The second method for Wizards to add spells to their spellbook is from their prepared spells.

If you lose your spellbook, you can use the same procedure to transcribe the spells that you have prepared into a new spellbook.

You don't prepare your Bard spells, so this method won't work either. Note that even if you did prepare Bard spells:

You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.

So even if you did prepare Bard spells, they wouldn't be considered to be prepared by the Wizard class and wouldn't be usable with Wizard class features, including scribing.

With all that said, you should be able to craft a spell scroll of Comprehend Languages as a Bard (Crafting a Magic Item, DMG pg. 128) and then copy it into your spellbook from there.

A wizard spell on a spell scroll can be copied just as spells in spellbooks can be copied.

However, both the rules on finding spells and the rules on spell scrolls agree that the spell must be a "wizard spell". Unfortunately, the rules never define what it means for a spell to be a wizard spell, or a bard spell, or any other class's spell. The best evidence I can find is this:

Each time you gain a wizard level, you can add two wizard spells of your choice to your spellbook for free.

The only interpretation I can come up with of this that makes any sense is that wizard spells just means spells on the wizard list. This would mean that spells common to both the can be scribed by a Bard and then copied by a Wizard.

This strategy is reliant on some ambiguous points, and requires explicit DM permission, since it's up to them whether you can craft items or not. There are also additional conditions to copying spell scrolls written in the DMG entry for spell scrolls.

Rules As Written: It's unclear

Rules As Intended: You likely know which spell it is

5th edition has tried to stay away from "GOTCHA!" moments, so it feels against the spirit of the game that they would make you spend time and money, only to find out that you don't want to copy "Pete's Portable Outhouse" into your spellbook, or whatever.

There are various modules wherein players find a spellbook "with X,Y, and Z spells" and no mentions of "after the wizard deciphers part of the spellbook, they realize that these are the spells in the book."

Additionally, that you could potentially start trying to copy down a spell that you can't prepare (a level 2 spell, while you're still level 1) would potentially be possible if you didn't know what the spell was. Would that mean that you've wasted the 2 hours per level and 50 GP per level up to what you could potentially prepare? Would the DM just tell you "You try to decipher the spell and fail."? Neither of those seem to be keeping with game intent.

Copying a Spell into the Book. When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a level which you can prepare and if you can spare the time to decipher and copy it. (emphasis mine)

Rules As Fun (and Rules As Common Sense, likely): You definitely know what the spells in the book are

It is neither fun, nor interesting, nor interactive in any way to make players decipher what the spell is before they can decide whether or not they want it.

Just imagine being a first-level wizard and your DM telling you "You find a dusty tome in the abandoned wizard's tower. You have to spend at least 50 gold pieces each to find out what the ten spells within can do."

You'd sell the book for what you could and walk away (or at least I would).

...

Another interaction occurs to me. If you want to pay to copy a spell out of a wizard's book, you wouldn't actually know what spell you were copying until you were done. While hilarious once, it would be a huge jerk move on both the side of the wizard and the DM allowing it.