Problems with national letters in file names.

[radpopl]

If the file has a name like "ąćęłńóśżź" or "Gdańsk", ffms2 cannot open such a file (different symbols will appear in place of national symbols).
ffms2("Gdańsk.mp4",atrack=1)
I think the last good version was 2390, which is why I still use it (LWLibavVideoSource also has no problem with this).
If this information is not enough, I will make screenshots.

[qyot27]

Switch the Windows code page to UTF-8.

[radpopl]

Switch the Windows code page to UTF-8.

You mean convert the avs file to UTF-8? But this is just a workaround.

[qyot27]

I mean switching the system code page (aka 'system locale') to UTF-8.

In April 2018 (or possibly November 2017[13]), with insider build 17035 (nominal build 17134) for Windows 10, a "Beta: Use Unicode UTF-8 for worldwide language support" checkbox appeared for setting the locale code page to UTF-8.

An actual screenshot of classic AviSynth 2.6 being fine with it, nearly five years ago:
https://forum.doom9.org/showthread.php?p=1909790#post1909790

There are also ways for individual programs to set UTF-8 as their code page while leaving the system in a non-Unicode locale, but when the system-wide setting will usually work fine, there's little reason to worry about that. If there are some programs that don't like the system locale change, then addressing it by having the exact host program opening the script use the right UTF-8 manifest thing would accomplish the same goal.