There is an official answer to this question.
This post contains spoilers. Make sure to be caught up with the manga (currently Chapter 110).
First, let us lay the groundwork for the answer:
Eldian is the official language of Eldia, Marley, and many other nations around the world.
As a result of the old Eldian Empire's imperialism, Eldian is a known language throughout the world. It is spoken both inside and outside the Walls, although their writing systems differ. Ambassadors from across the globe used it to communicate with Willy Tybur during his pre-party.
There are three known writing systems for Eldian:
An archaic version of Eldian is found in ancient manuscripts and texts
that were sent by Eren Kruger to the Eldia Restorationists. The
language appears to include a lot of visuals and was used by the
Eldians from a long time ago. Although it was unreadable to the
Grisha Yeager and Restorationists, he and his fellow patriots assumed
that the texts confirm their beliefs that Ymir Fritz had brought
wealth to mankind, and that the Subjects of Ymir are the chosen
children of God.
Inside the Walls
This system of writing is found only inside the Walls. The writing is
composed by short, straight strokes and sharp corners, and written
Outside the Walls
Another system of writing is found when the Survey Corps settled in
Utgard Castle after searching for a supposed breach in Wall Rose. They
found supplies that were left behind by unknown occupants, but the
labels found on them were unreadable to most of the soldiers. When
Ymir went off to find food, she found a can with a herring label on it
which she proceeded to read out loud. She gave the can to Reiner
Braun, who was shocked to find out that she could read the writing. It
is later revealed that both Reiner Braun and Ymir came from Marley,
confirming that the writing is the system used by the Marleyans and
the Eldians living outside the Walls.
Now regarding your question: According to the Attack on Titan Guidebook,
Reiner Braun could read the herring can like Ymir, but he lied in order to protect his own identity while forcing Ymir to reveal hers.