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Data can be stored in UTF-8 format when working with the WSTRING type in the IEC code.
By using the ConvertUTF16toUTF8 function from the StringUtiIs library, the text is converted to UTF-8 format and can be saved to a file.

Create a "Standard project" and select CODESYS Control Win V3 as the device.
Define the target system by means of the Network scan.
Open the Library Manager and add the following libraries:
StringUtils
SysFile
SysTypes2 Interfaces

Edit the PLC_PRG POU as follows:

Declaration

VAR
    xWrite         : BOOL;
    szFileName     : STRING(128) := 'c:/tmp/TestUTF8.txt';
    hFile          : SysTypes.RTS_IEC_HANDLE := SysTypes.RTS_INVALID_HANDLE;
    iecResult      : SysTypes.RTS_IEC_RESULT;
    wsTextLine     : WSTRING(10) := "Übersetzer";
    sUTF8TextLine  : STRING;
    udiPos         : __XWORD;
    udiWrite       : __XWORD;
END_VAR

Implementation

IF xWrite THEN
    xWrite := FALSE;
    hFile := SysFileOpen(szFile:=szFileName, am:=SysFile.AM_Write , pResult:=ADR(iecResult));
    IF hFile <> RTS_INVALID_HANDLE    THEN
        Stu.ConvertUTF16toUTF8(sourceStart := ADR(wsTextLine), targetStart := ADR(sUTF8TextLine), dwTargetBufferSize := SIZEOF(wsTextLine), bStrictConversion := TRUE);
        udiWrite := SysFileWrite(hFile:=hFile, pbyBuffer:=ADR(sUTF8TextLine), ulSize:=INT_TO_UDINT(LEN(sUTF8TextLine)), pResult:=ADR(iecResult));
        iecResult  := SysFileClose(hFile:=hFile);
    END_IF
END_IF

Start the project and set the xWrite variable to TRUE.

If the file is viewed in an editor, then the text is displayed correctly: