For the film title and all along the CPL/DCP name you need to refrain from using anything else than the English alphanumeric characters. Excluding accents, acute or grave, circumflexes, centiles, umlaut, diaeresis, punctuation like the apostrophe and (the less obvious) i with no puncture (dot), etc., etc. Same goes for languages that use additional characters, like the double s in German, and I can go on more than necessary to make a point.
It's not only the number of the characters that DCP-o-matic will allow you to use, it might as well mess up the Media Block of the cinema server at the cinemas. I know first hand what I am writing about. An "í" (you see the acute accent?) that passed by the DCP maker of Premiere Pro obliged me to stop, delete the DCP and reboot both the server and the projector (Integrated Media Block), in order to restore the system's ability to work.

The DCP folder name, and -if used- in the .mxf files is the one from the CPL (Composition Playlist) "ContentTitleText", and not the other way around.
In other words, the restrictions that govern the xml tag <ContentTitleText> are/may affect the file/folder names, that otherwise might be named in another fashion (by the UUID of the CPL/MXF, or even more randomly).
So, if by "metadata" you refer to the CPL values and the way a DCP appears on the cinema-server, unfortunately, we are restricted from having a value such as "T'as pas changé".

https://cinema-int.com/CNCN/index.html does not underline the problem in question.

In regards to having more than 14 characters for the title, you may do that on DCP-o-matic if you un-check the ISDCF compatibility and edit the pre-set name.
I do that once in a while, not for extra characters, but to alternate capital and minuscule letters etc. after I have set all other parameters on the name (like language, subtitles, etc. etc.)
That, though, doesn't solve the so called " Le Problème " on that link I gave you earlier.

While that automatic dropping of characters may make „maður“ ("man" in Icelandic) to become "maur", it is a failsafe I am glad is there.

Could it be that by "metadata" you mean something else that the CPL entries?

The early truncation with accents/apostrophe should be fixed in 2.18.35

@carl 054e20e8fbc04a9a7bb78b2c4a52f9ca9c817df2

Capitalising after an apostrophe is a bit tricky as it is language-dependent (you wouldn't want it in English, normally, for example).

I could see an argument for allowing a different ContentTitleText to avoid the 14-character limit. There is this FullContentTitleText in the extra SMPTE metadata, which seems to have been added because everybody puts the ISDCF name into ContentTitleText. DCP-o-matic will put the plain film name into FullContentTitleText.

Sadly I don't know which thing systems in the field present to the user... I suppose it's the CPL's ContentTitleText.

@IoannisSyrogiannis it seems that the crash you mentioned was due to the non-ASCII i in the filename, not the the XML metadata?

It's been a few years now, since May '21, I am not sure I can find the necessary info any more. I looked for logs, but what I have is a few months later. I have some correspondence saying that an Alchemy would not ingest and a (Dolby) DSS200i would eventually ingest via network but go brain-dead when pressing play. Re-packaging with DCP-o-matic did the work.
I remember that it was a wraptor (Premiere) export and I seriously doubt that the filename (MXF file) included the DCP/project name.
The servers are often enough replacing the CPL file name, as well as the PKL name. If I were to guess, I would guess that the problem was with one (or all) the TitleText tags.