[Fontconfig] ISO 15924 font selection

[Gerrit Sangel]

Hello,

Sorry if this was asked before, but I could not find anything in the archive.

Has fontconfig the option to define fonts according to ISO 15924 (and not only 
according to ISO 639)?

If not, I would propose this option:

In my opinion, it is much more flexible than defining fonts according to a 
specific region (e.g. TW or CN). In some cases, it is even necessary, because 
the region does not differ.

Do I understand this correctly, that the user can specify a font in the config 
file according to a specific language?
I see this in Firefox (even though it does not seem to use fontconfig, but I 
guess an addon could be written to solve it), that I can specify fonts 
according to language (e.g. Chinese Traditional (Hongkong)) and the Browser 
selects the font if the html file includes the xml:lang attribute. But this 
is a bit inconvenient to do this in every application, so I guess fontconfig 
changes this globally?

But I only saw in my config files options to generally define an order of font 
substition, but not according to language and script tags?

Returning to my question:

German Fraktur has the ISO 15924 tag “Latf”, which is necessary to define that 
a paragraph should be displayed in a Fraktur style (because it uses the same 
code points as normal Latin characters). But there is no region to define, so 
I guess the correct tag would be “de-Latf”. But there are no options anywhere 
to specify a font for this case.
Another possibilities would be e.g. “ja-Latn” for Japanese in Latin 
transcription.

Is it maybe possible to implement this?
I guess, a flexible way for this would be:

Latn (generally): DejaVu Sans
Latf (generally): Breitkopf Fraktur
ja (generally): Kochi Gothic
ja-Latn: DejaVu Sans
ja-Hant: some font with old glyphs

and so on.
So I think a possible way would be to define a general rule for a language 
(according to ISO-639) or a script (ISO 15924) at first and then a specific 
rule for a language or script which would override the general rule.

Thanks
Gerrit