Citations in two different languages for the same LaTeX document

Question

I am writing a LaTeX document using overleaf for which I need to write in both french and english. I am using the \citalp function from the natbib package, which makes citations as follow:

• Smith, 1999. If 1 author
• Smith and Jones, 1999. If 2 authors.
• Smith et al., 1999. If more than 2 authors.

But when I am writing in french, i need the citation to be "Smith et Jones, 1999" for the two authors case, "et" standing for "and". But I still need to keep \citalp for english paragraphs.

Here is an example of what I have:

TeX file:

\documentclass[a4paper,12pt]{article}
\usepackage{natbib}
\begin{document}
\section{French section}
Let's say this is french text for which i need to cite "Smith et Jones, 1999" but sadly, the citealp function gives me this: \citealp{smith1999}
\section{English section}
This paragraph is written in English so using citealp is what i need:  \citealp{smith1999}
\bibliographystyle{plainnat}
\bibliography{test}
\end{document}

test.bib

@article{smith1999,
    author = {Smith, A. and Jones, B.},
    title = {A very nice paper},
    journal = {Great Journal},
    year = {1999}
} 

Answer 1

If you don't have to absolutely use the natbib-package, I would recommend a rather painless switch to biblatex (with natbib=true). moewe is absolutely right here, this change will make your life so much easier.

There are a couple of things which have to be changed, like the removal of the small caps for names in French (see the comment in my MWE) and a couple of initials. This comes pretty close to natbib and will infinitely simplify your life.

I have tested the following minimal working example using overleaf. It worked out of the box:

\documentclass[a4paper,12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
% Bibliography-Settings
\usepackage[
style=authoryear,giveninits=true,uniquename=init,natbib=true
]{biblatex}
% Remove small caps in the French version
\DefineBibliographyExtras{french}{\restorecommand\mkbibnamefamily}
\addbibresource{test.bib}
\usepackage{csquotes}
\usepackage[french,english]{babel}
\begin{document}
\section{French section}
\selectlanguage{french}
Let's say this is french text for which i need to cite "Smith et Jones, 1999" but sadly, the citealp function gives me this: \citealp{smith1999}
\section{English section}
\selectlanguage{english}
This paragraph is written in English so using citealp is what i need: \citealp{smith1999}
\printbibliography
\end{document}

Here is what it looks like: