What is the best way to identify the codepage of a file? I hoped i can use DetectInputCodepage but i dont know how to implement it with progress.
I don't know what led you down that rabbit hole - the link you provided led to outdated documentation for Internet Explorer.
However. This problem is non trivial. See https://community.progress.com/s/article/000057930
Also
How can I detect the encoding/codepage of a text file?
Based on the answer above (on Stackoverflow) you can download the mentioned Nu-package and extract the dll (using any zip-tool).
Add the ude.dll-file to your assemblies and then you can do like this (error checking etc missing in this example):
USING Ude.*.
USING System.IO.*.
DEFINE VARIABLE fs AS FileStream NO-UNDO.
DEFINE VARIABLE ude AS CharsetDetector NO-UNDO.
fs = NEW FileStream( "c:/temp/test.txt", FileMode:Open).
ude = NEW CharsetDetector().
ude:Feed(fs).
ude:DataEnd().
IF ude:Charset <> ? THEN
MESSAGE SUBSTITUTE("Charset &1, confidence &2", ude:Charset, ude:Confidence) VIEW-AS ALERT-BOX.
ELSE
MESSAGE "Failed" VIEW-AS ALERT-BOX.
fs:Close().
DELETE OBJECT fs.
DELETE OBJECT ude.