Why babel produce warnings about \rmfamily and \ttfamily with \babelfont

Question

I have babel v3.34 a simple example with beamer which I have added a sans serif font family with \babelfont{sf}[Language=Default]{DejaVu Serif} produce such warnings

Package babel Warning: The following fonts are not babel standard families:

• \rmfamily / lmr [lmroman10-regular]:+tlig; at 10.95pt
• \ttfamily / lmtt [lmmono10-regular]: at 10.95pt
  There is nothing intrinsically wrong with it, but 'babel' will no set Script and Language. Consider defining a new family with \babelfont.

\documentclass{beamer}
\usepackage[english]{babel}
\babelfont{sf}[Language=Default]{Deja Vu Serif}

\begin{document}
\begin{frame}{}
Test 
\end{frame}

\end{document}

Why I need to add a roman font family and a teletype font family?

( equivalent MWE with fontspec produce result without any warnings )

@JavierBezos

Answer 1

The main purpose of \babelfont is to define at once in a multilingual document the fonts required by the different languages, with their corresponding language systems (script and language). So, if you load, say, 4 languages, \babelfont{rm}{FreeSerif} defines 4 fonts (with their variants, of course), which are switched with the language by babel. It's just a tool to make things easier and transparent to the user.

There is no real need to use \babelfont in a monolingual document, if you set the language system in \setmainfont (or not, depending on what you want).

babel assumes (actually, I assume :-)) that if you are using \babelfont for a family, very likely you want to define the rest of them. If you don't, you can find some inconsistencies between families. This checking is done at the beginning of the document, at a point where we cannot know which families will be used.

As the message explains, there is nothing intrinsically wrong with not defining all the families. In fact, there is nothing intrinsically wrong with not using \babelfont at all. But you must be aware that this may lead to some problems. And this is the very reason of the warning.

Answer 2

The package babel is informing you that you lack definitions for the serif family and the mono family.

\documentclass{beamer}
\usefonttheme{professionalfonts}
\usepackage[english]{babel}
\babelfont{rm}[Language=Default]{DejaVu Serif}
\babelfont{sf}[Language=Default]{DejaVu Serif}
\babelfont{tt}[Language=Default]{DejaVu Sans Mono}

\begin{document}
\begin{frame}{}
Test
\end{frame}

\end{document}