Thanks to Thomas and Denis,

That's it: use GFS Didot for the French text and Theano Didot for the 
Greek. In my working text I have this preamble (and other things):

\setuplanguage[fr][patterns={fr,agr}]
\mainlanguage[fr]
\setcharacterspacing[frenchpunctuation]]
\setuplayout [backspace=40mm]
\setuppagenumbering[alternative=doublesided, location={header, inmargin}]
\setupmargindata[inmargin][location=inner, 
style={setupinterlinespace[line=2.8ex]}]
\setupmarginframed[inmargin][align=right]
\setupbodyfontenvironment[default][em=italic]
\definefallbackfamily[mainface][rm][Theano 
Didot][preset=range:greek,force=yes]
\definefontfamily[mainface][rm][GFS Didot]
\setupbodyfont[mainface, 11pt]

I'm currently working on MS Windows 11 for the moment (as a test beside 
Linux). I didn't try to compile this text under Linux, but I don't think 
there is an issue with the OS.

As far as GFS Didot is concerned, I have installed the most recent 
version. But what amazes me the most is that by only keeping a font like 
EBGaramond without writing :
\definefallbackfamily
  [mainface]
[rm]
[Theano Didot]
[preset=range:greek,force=yes]
So, using EBGaramond only, I have the French text (with italics), as 
well as the Greek text...

Try this :

\setuplanguage

\mainlanguage[fr,agr]

\definefontfamily

[mainface]

[rm]

[EBGaramond]

\setupbodyfont[mainface, 12pt]

\setupbodyfontenvironment[default][em=italic]


\starttext

Socrate est descendu au Pirée et affirme sa satisfaction du culte rendu 
à cette nouvelle déesse par les Thraces; néanmoins, {\em il ne nous dit 
rien des prières} qu'il a lui-même rendues à la déesse avec les autres 
participants, dont il fait pourtant état. Sur le chemin du retour, il 
est arrêté par Polémarque qui leur apprend qu'une retraite aux flambeaux 
à cheval est prévue par les Thraces, ainsi qu'une fête de nuit 
\quote{qui vaut la peine d'être vue} (ἣν ἄξιον θεάσασθαι; {\em èn 
axion théasasthai} - 328a).

\blank


Κάθε άνθρωπος δικαιούται να επικαλείται όλα τα δικαιώματα και όλες τις 
ελευθερίες που προκηρύσσει η παρούσα Διακήρυξη, χωρίς καμία απολύτως 
διάκριση, ειδικότερα ως προς τη φυλή, το χρώμα, το φύλο, τη γλώσσα, τις 
θρησκείες, τις πολιτικές ή οποιεσδήποτε άλλες πεποιθήσεις, την εθνική ή 
κοινωνική καταγωγή, την περιουσία, τη γέννηση ή οποιαδήποτε άλλη κατάσταση.


Δεν θα μπορεί ακόμα να γίνεται καμία διάκριση εξαιτίας του πολιτικού, 
νομικού ή διεθνούς καθεστώτος της χώρας από την οποία προέρχεται κανείς, 
είτε πρόκειται για χώρα ή εδαφική περιοχή ανεξάρτητη, υπό κηδεμονία ή 
υπεξουσία, ή που βρίσκεται υπό οποιονδήποτε άλλον περιορισμό κυριαρχίας.

\stoptext


I've found a solution, with Segoeui (Windows font) as the main font and 
Theano Didot for the Greek text.
But I don't understand why setting the fonts to GFS Didot and Theano 
Didot prevents italics from being displayed ...


Best//JP





Le 26/02/2024 à 15:02, Thomas A. Schmitz a écrit :
> I just tried your example with the latest version of GFSDidot (the older version I had on my system did not show some diacritics; this problem disappeared when I installed the latest), and the italics are there, I don’t see any problem in the pdf. If you want the Greek displayed in Theano, you will have to add “force=yes” to your definition of the fallback family:
>
> \definefallbackfamily
>      [mainface]
>      [rm]
>      [Theano Didot]
>      [preset=range:greek,force=yes]
>
> Your text cannot have two main languages, so
>
> \mainlanguage[fr]
>
> If you want hyphenation for your Greek passages, you can add
>
> \setuplanguage [fr] [patterns={fr,agr}]
>
> The line where you have polytonic Greek does not use precomposed characters, but combinations of greek characters and diacritics; this will probably not work in recent context versions that expect actual unicode input.
>
> I hope I have really understood what you want: GFSDIdot for the French text and Theano for Greek?
>
> HTH
>
> Thomas
>
>> On 26. Feb 2024, at 13:32, Jean-Pierre Delange<adeimantos@free.fr>  wrote:
>>
>> Dear list,
>>
>> When handling the preamble to a document in which I would like to treat the text in French, with sometimes italics text and phrases in ancient Greek, I encounter the following difficulty:
>> if I use TeX Gyre Pagella as the main font, everything works fine, except for the Ancient Greek text, which I don't like the look of. But the Greek text is displayed, as are the French italics. On the other hand, if I use GFS Didot as the main font and Theano Didot for the Greek, the italics disappear and the Greek text looks better. The CTX version I use is: mtx-context | current version: 2024.01.24 22:39.
>>
>> Herewith are two MWE attachments (*.tex + *.pdf) with different fonts. What am I missing ? Thanks for your answers.
>>
>> JP
>
>
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