ADDITIONAL SYSTEM INFORMATION :
Windows 11 JRE 21.0.1

A DESCRIPTION OF THE PROBLEM :
For the past twenty years, I have been developing Java applications whose work files include, for the French version, special characters such as accents or others.

So far, I've had no problem reading them with earlier versions of Java (I personally did it until Java 15), but these special characters now appear unreadable.
If you'd like to download the application with which I encountered these problems, you can find various versions of it here:
<LINK>
I ran the tests with demo versions 2.52 and 2.82 on Windows. The main problems appear when you load one of the working files after installing the software package.
The launch is done by ThoptExec.jar

REGRESSION : Last worked in version 15

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Download the French version of the application: <LINK>
I ran the tests with demo versions 2.52 and 2.82 on Windows.
Install the software package. The launch is done by ThoptExec.jar
A first window asks you to accept the license. Sometimes illegible characters appear.
When you accept, two JFrames are opened, the simulator and the diagram editor.
Load files of the same names in both of them (for instance TAG.prj and TAG.dia)


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
With previous versions of the JRE the special characters were properly displayed.
ACTUAL -
Some illegible characters appear.

---------- BEGIN SOURCE ----------
The source code is about 100 000 lines and not public
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Tell users to use an old JRE version

FREQUENCY : always