I have this example

documentclass[hyperref=unicode]{beamer}



usepackage[english,nil,bidi=basic-r]{babel}

babelprovide[import=ar-DZ, main]{arabic}

babelfont{rm}{Amiri}

babelfont{sf}{Amiri}





begin{document}



begin{frame}{}

today

end{frame}



begin{frame}{}

selectlanguage{english}

today

end{frame}



end{document}

Compiled with lualatex I obtain this warning

Package fontspec Warning: Language 'English' not available for font
'Amiri'

What I have done wrong? or what I have missed?

This bugged me for a long time too. This is what is going on:

These are the scripts and languages supported by Amiri:

DFLT        Default

arab        Arabic

arab.ARA    Arabic/Arabic

arab.KSH    Arabic/Kashmiri

arab.SND    Arabic/Sindhi

arab.URD    Arabic/Urdu

latn        Latin

latn.TRK    Latin/Turkish

When you load Arabic with babelprovide[import=ar-DZ, main]{arabic} then babel reads in babel-ar-DZ.ini.

In this file you will find (among other things):

tag.opentype = ARA

script.tag.opentype = arab

So when Algerian Arabic is the language in use, babel will load Amiri with arab.ARA script and language. This exists in the font, so all is good.

But babel-en.ini contains this:

tag.opentype = ENG

script.tag.opentype = latn

So when English is in use, babel will try and load Amiri with latn.ENG script and language. Except this combination does not exist in the font, so a warning is issued by babel:

Language 'English' not available for font 'Amiri' with script 'Latin'.

Now all we need to do is tell babel to use Language=Default with Script=Latin for English while continuing to use Language=Arabic with Script=Arabic for Algerian Arabic.

To do this, remove english from the main babel options and instead load it like this:

babelprovide[import,language=Default]{english}

Note: babel uses language with a lowercase l as opposed to fontspec which uses Language!

babel will now load Amiri with supported options for English text and no warning is issued.

MWE

documentclass[hyperref=unicode]{beamer}



usepackage[nil,bidi=basic-r]{babel}

babelprovide[import=ar-DZ, main]{arabic}

babelprovide[import, language=Default]{english}

babelfont{rm}{Amiri}

babelfont{sf}{Amiri}    



begin{document}



begin{frame}{}

today

end{frame}



begin{frame}{}

selectlanguage{english}

today

end{frame}



end{document}

You can set the language to default:

documentclass[hyperref=unicode]{beamer}



usepackage[english,nil,bidi=basic-r]{babel}

babelprovide[import=ar-DZ, main]{arabic}

babelfont{rm}[Language=Default]{Amiri}

babelfont{sf}[Language=Default]{Amiri}





begin{document}



begin{frame}{}

today

end{frame}



begin{frame}{}

selectlanguage{english}

today

end{frame}



end{document}

Nothing wrong, on the contrary. These warnings are shown by fontspec, not by babel. They could be irrelevant for English, but not for many other languages, including Arabic. To remove them altogether (they are only really useful when the document format is being set up), you may pass the silent option to fontspec:

usepackage[english,nil,bidi=basic-r]{babel}

babelprovide[import=ar-DZ, main]{arabic}

PassOptionsToPackage{silent}{fontspec}

babelfont{rm}{Amiri}

babelfont{sf}{Amiri}

Or also:

usepackage[english,nil,bidi=basic-r]{babel}

babelprovide[import=ar-DZ, main]{arabic}

usepackage[silent]{fontspec}

babelfont{rm}{Amiri}

babelfont{sf}{Amiri}