You'd have to modify the style -- the consensus among Polish translators was that et al. is used more commonly than i in, so that's what's in the Polish translation.

To change that, you'll see a whole number of small sections that change the et al definition in citation style, looking like this
<locale xml:lang="da">
<terms>
<term name="et-al">et al.</term>
</terms>
</locale>

You'd just add one for Polish in the same place, so

<locale xml:lang="pl">
<terms>
<term name="et-al">i in.</term>
</terms>
</locale>

https://www.zotero.org/support/dev/citation_styles/style_editing_step-by-step

You can make the style available to your students -- we can probably also host it on the repository if you want to submit it there under the name of your university https://github.com/citation-style-language/styles/blob/master/CONTRIBUTING.md