Comprehend Languages allows you to read and understand spoken language, but not speak or write.
Tongues allows you to speak and comprehend spoken language, but not read or write.
Is there anything (spell, class ability, magic item, etc.) that would allow a creature to write in an unfamiliar language (possibly in conjunction with one of the above spells)?
The Belt of Dwarvenkind (DMG p. 155) allows the attuned wearer to speak, read and write Dwarvish.
It you're willing to delegate, there's a mundane solution: a scribe is a skilled hireling who will charge 2 gp per day for their services (PH p. 159), and you can check their work is reliable with comprehend languages.
With only 250 gold pieces and 250 days steeped in training, you can learn a new language between adventures, according to PHB p187 under "Training".
As far as I know and read, there is nothing that allows to write in an unfamiliar language directly, but there are some workarounds.
If you know the race this is your best bet: you can polymorph* into a creature of that race and write in the language of your new form
The target's game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
This is the most direct way of writing in another language that I can think of.
*- Crawford twitted a comment on True Polymorph and languages. Also, there is this question about language.
If transformed by the true polymorph spell, the creature would speak what the new form speaks.
For a more permanent solution but as risky as a moody DM can be, the spell Wish can make you "learn" a new language (with all its implications). For a "risk-free" way wish it to be temporal.
I was tempted of not including this one since it is obvious, but there is a particular spell that can help you in a situation where everyone is your enemy: Dominate Person. Fetch yourself a creature with the language that you desire, apply the spell and watch your new assistant do all the work for you. The downside is that it wont be able to write you a book since the duration is up to 8 hours spending an 8th level spell slot.
While the target is charmed, you have a telepathic link with it as long as the two of you are on the same plane of existence. You can use this telepathic link to issue commands to the creature while you are conscious (no action required), which it does its best to obey.
Also, spells like Conjure Celestial are alternatives if the particular language are within any race that can be summoned and can write.
The only thing I've been able to find doesn't explicitly allow you to write an unfamiliar language but it does allow you to read it. And if you can read it, what's stopping you from writing the same thing you read. The feature I'm talking about here is the Warlock Eldritch Invocation "Eyes of the Runekeeper" which simply says that "You can read all writing"
