Vietnamese warnings with babel and biblatex

[Rimole]

Copy from bug report at tex.stackexchange (not my report, but found it when I encountered the same issue):

\documentclass{article}

\usepackage[T5,T1]{fontenc}
\usepackage[vietnamese, british]{babel}
\usepackage{csquotes}
\usepackage{biblatex}

\begin{document}

\foreignlanguage{vietnamese}{Ngô Bảo Châu}

\end{document}

The above code compiles in PDFLaTeX, but I get two warnings:

Package babel Warning: No input encoding specified for Vietnamese on input line 146.
...
Package biblatex Warning: Language 'vietnamese' not supported.
(biblatex) Using dummy definitions on input line 8.

How can I avoid these two warnings?

The first can be removed with \usepackage[utf8]{inputenc}, but I feel like it isn't the correct way: this line should be nowadays obsolete, and there should be a more direct way to tell babel the correct encoding for Vietnamese.

asked Jul 13 at 9:56 by Gargantuar at https://tex.stackexchange.com/q/722463

Comment by David Carlisle:

as far as I can see your workaround for inputenc is what it needs to be (or patch internal commands) you should report it to babel as a bug the file should be updated to check that utf-8 is enabled not just check if inputenc is loaded. for biblatex I think you need to make a lbx file following these steps https://github.com/plk/biblatex/wiki/Checklist-for-submitting-a-new-localisation-file-(.lbx).