The main issue on this - "foreign characters valid in OS file names" is not addressed...

It's just plain ugly for native readers in those languages to replace the characters with expanded "regular" chars..., like:

Ü  UE, Ö OE, Ä AE, ß ss

The text is in German and there is something called "Umlaute" a -> ä, u -> ü etc. and those are rejected by piwigo.

It is not necessary to read and understand the text in the screen shot I posted, just the fact that those characters are rejected in file names by piwigo when files or (maybe) titles are rejected and one has to put those workarounds in ä -> ae  etc. which is third class text IMO and makes it harder to read.

Why piwigo cannot handle those texts and rejects them whereas the OS and programs in there are localized.

Piwigo allows other localities but fails in this instance.

In addition, putting those files with "Umlaut" names from a German native system to piwigo, one has to go manually in and change them all to those double character replacement to the images and maybe texts (?) will be accepted. 

It is not fully localized or this issue would not come up.

The screenshot shows piwigo's error message rejecting the names.

Is it more clear then?

1.) Copy directory tree with German characters in file name on the server under Piwigo's picture tree
2.) Change permissions within directory tree of concern on server to piwigos conventions
3.) Under Admin "Synchronize" all

You get the error message shown in the attachment.  I have translated the text back to EN.

The error message that the folder/file does not exist or is not accessible is incorrect and misleading.
When piwigo encounters a folder/file name with characters "öäü?ÖÄÜ", it cannot recognize it and issues the error messages shown in the screen shot.



DE:
                               Fehlerliste
[./galleries/DE/Von-Rosi/18D325_Gruß_vom_Frühling] PWG-UPDATE-1 (falscher Dateiname)
[./galleries/DE/Von-Rosi/190424-Osterbrunnen_in_Wörishofen] PWG-UPDATE-1 (falscher Dateiname)
[./galleries]DE/Von-Rosi/181224-Weihnachsgrüße] PWG-UPDATE-1 {falscher Dateiname)

                           Fehlerüberschrift
PWG-UPDATE-1: Verzeichnis- und Dateinamen müssen aus Buchstaben oder den Zeichen "-", "_" oder "." bestehen.
PWG-ERROR-NO-FS: Auf diese Datei/ dieses Verzeichnis kann nicht zugegriffen werden. Entweder existiert die Datei/ das Verzeichnis nicht
verweigert.


EN:
                                 Error List
[./galleries/DE/Von-Rosi/18D325-Gr®vom_F®lin | PWG-UPDATE-1 (wrong filename)
[./galleries/DE/Von-Rosi/190424-0st-erbrunnen '@ hofen] PWG-UPDATE-1 (wrong filename)
[.1 galleries] DE / By- Rosil 181224-Christmas :: h PWG -UPDATE- 1 (incorrect filename)

                                Error Heading
PWG-UPDATE-1: Folder and file-names have to made from letters or the characters "-", "_" or ".".
PWG-ERROR-N0-FS: This folder/ file can can not be accessed. Either the file/ folder does not exist (missing text) denied.

This is a bug:
Error messages are wrong.
Software is not localized to set locality and does not recognize localized file system.