Which language is used to program a PLC?

If you are in the USA the main "programming language" as such is called "ladder logic" it is sort of like a combination of electrical drawings and boolean logic.

This site has a good tutorial. https://www.plcfiddle.com/

In other locales, they may lean more on something called "structured text"

The languages are supposed to be standardized, but much like old browser HTML support, each major company's implementation may differ by a little, but once you get the basics you should be fine.

IEC standard 61131-3 lists 5 programming languages for PLCs.

Ladder diagram sequential flow chart functional block diagram structured text instruction list

Ladder is very common. It is essentially an electrical diagram showing the relay-relay logic circuit used before PLCs became popular. it has a source on the left and on horizonal rungs there will be various contacts leaving to a output on the right side. the series of rungs looks like a ladder.

Sequential flow charts look more like a state machine. What condition causes the state to change from one state to another. The actual states and the condition that causes the translon are usually written in one of the other languages. This not commonly used except in some large complicated programs.

Functional block diagram more closely resembles a logic circuit made of IC chips. Each block has inputs and outputs. a particular set of inputs cause a set of outputs. It is also fairly common and handles more complex programs than can easily be written in ladder.

Structured text is not really used much. think of an early programming language like BASIC.

Instruction list is even more primitive. I usually compare it to writing your code in assembler.

So most use ladder, FBD, or some combination of both. Possibly with SFC as a top layer.