How can I use an external multiline text editor?

Symptoms

MTEXTED does not accept the WordPad text editor and returns: "The specified value is not within the system variable range".
It is not possible to enter international characters by pressing a combination of two or more keys - for instance, Japanese characters using IME, or Latin characters such as É, ã...

WordPad is often not in the environment variable %PATH% so BricsCAD does not find it while Windows can still find it.
The MTEXT in-place editor needs to properly process all possible character input events. This is a difficult task, especially on Linux because of the wide choice of distributions and desktop managers.

This can be solved by either extending the SRCHPATH variable with the path to wordpad.exe or by filling in the full path to the application in the MTEXTED system variable. (e.g., C:\Program Files\Windows NT\Accessories\wordpad.exe)
As an alternative to the in-place MTEXT editor, you can change the value of the _MTEXTED variable. You can either enter an external executable for text editing, or you can enter the value oldeditor to show a built-in MTEXT edit dialog instead of the MTEXT in-place editor. The edit dialog contains a standard text edit field, which typically does not suffer from international character input issues.

This is a workaround to be able to continue working without delay. Be sure to report your character input issue anyway, so we can fix the in-place MTEXT editor as well.

Note: It is recommended to use the standard in-place editor.