Use Latin language with babel

Question

I'd like to typeset some parts of the text in Latin language (preferably New Latin, if possible). Latin language is listed among supported languages of Babel. I invoke it as \usepackage[main=british, french, german, latin]{babel} and use in the text like \begin{otherlanguage}{latin}TEXT\end{otherlanguage}, but XeLaTeX keeps telling me Language 'latin' not supported'. This works well for the other languages I use. How can I use Latin language?

As comments suggest, the warning can be produced

\usepackage[
backend=biber,
bibstyle=authoryear,
citestyle=authoryear-comp,
sorting=ynt,
autocite=inline,
maxbibnames=99,
mincitenames=1,
maxcitenames=2,
hyperref=true,
backref=true,
backrefstyle=none,
useprefix=true,
uniquelist=false
]{biblatex}
\DeclareFieldFormat[article]{title}{\mkbibbold{#1}}
\preto\fullcite{\AtNextCite{\defcounter{maxnames}{99}}}

I have some bibliographic entries written in Latin, but the bibliography is typeset in English.

I use texlive 2018.151 on openSUSE Linux. All Latin related packages (well, I have all packages installed) are installed.

Answer 1

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[main=british, french, german, latin]{babel}
\usepackage{csquotes}

\usepackage[style=authoryear, backend=biber]{biblatex}

\addbibresource{biblatex-examples.bib}

\begin{document}
Lorem
\end{document}

produces the warning

Package biblatex Warning: Language 'latin' not supported.
(biblatex)                Using fallback language 'british' on input line 11.

Your editor may digest that message for you and may only display the 'important' Language 'latin' not supported. bit of that message prominently to you robbing you of the chance to see that it comes from biblatex.

Due to the way language loading works in biblatex, all declared document languages are loaded at the beginning of the document even if they are not used at all. Languages that are not supported yield a warning. Since there is no latin.lbx Latin is not supported by biblatex (if you want to contribute a .lbx file for an unsupported language or know someone who can, drop by at https://github.com/plk/biblatex/issues). If you do not plan to typeset citations or the bibliography in an unsupported language you safely can ignore that message.