"the 12th" is a correct and usable part of a sentence in written English.
You can use it when referring to a day of the month, such as "I will be arriving on the 12th".
Dictionary
the 12th
adjective
Abbreviation of twelfth
synonyms
Exact(60)
Since the 12th hole yesterday, he's -7!
The 12th factor did not include any meaningful (≥ .30) loadings.
Since the 12th cen.
And bogeyed the 12th hole.
Scafa led off the 12th inning.
By the 12th, he had killed Benny.
The 12th had Alzheimer's.
The 12th faints.
Dayib, the 12th, survived.
"The 12th was crucial," McGinley said.
That was in the 12th century.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com