When creating in-text citations CSL doesn't take into account localized terms for different languages and uses et-al term in a single language whether it's specified or not in Zotero in the language field
this results in [Author [et al.], 2019] for 1 language and [Auteur [et al.], 2017] for another language. But et-al terms are localized for different languages:
<locale xml:lang="en">
<terms>
<term name="et-al"> [et al.]</term>
</terms>
</locale>
<locale xml:lang="fr">
<terms>
<term name="et-al"> (et al.)</term>
</terms>
</locale>
<macro name="author_short">
<names variable="author">
<name form="short" name-as-sort-order="all" sort-separator=" " delimiter=", " delimiter-precedes-last="always"/>
<substitute>
<text variable="title"/>
</substitute>
</names>
</macro>
<citation et-al-min="4" et-al-use-first="1" disambiguate-add-year-suffix="true" collapse="year" et-al-subsequent-use-first="1" disambiguate-add-names="false" disambiguate-add-givenname="false">
<sort>
<key macro="language-sort"/>
<key macro="author-title"/>
<key variable="issued"/>
</sort>
<layout prefix="[" suffix="]" delimiter="; " page-range-format="">
<group delimiter=", ">
<text macro="author_short" suffix=""/>
<text macro="year-date"/>
</group>
<choose>
<if variable="locator">
<label variable="locator" form="short" prefix=", " suffix=" "/>
<text variable="locator"/>
</if>
</choose>
</layout>
</citation>
In CSL, the locale refers to the entire document/bibliography. It's not possible to have the localization of terms be determined by the bibliography item.
Zotero, specifically, does support the csl-m variant of CSL, in which you can do this by specifying separate layouts for all supported languages, so:
<macro name="citation">
<group delimiter=", ">
<text macro="author_short" suffix=""/>
<text macro="year-date"/>
</group>
<choose>
<if variable="locator">
<label variable="locator" form="short" prefix=", " suffix=" "/>
<text variable="locator"/>
</if>
</choose>
</macro>
<citation et-al-min="4" et-al-use-first="1" disambiguate-add-year-suffix="true" collapse="year" et-al-subsequent-use-first="1" disambiguate-add-names="false" disambiguate-add-givenname="false">
<sort>
<key macro="language-sort"/>
<key macro="author-title"/>
<key variable="issued"/>
</sort>
<layout locale="en" prefix="[" suffix="]" delimiter="; " page-range-format="" >
<text macro="citation"/>
</layout>
<layout locale="fr" prefix="[" suffix="]" delimiter="; " page-range-
<text macro="citation"/>
</layout>
</citation>
Do note that Zotero will warn you that the citation style is invalid on install and that it won't be accepted on the standard CSL repository.