If I switch the user language to, for example, https://es.wikipedia.org/wiki/Especial:Contribuciones/MarcoAurelio?uselang=$langcode different portions of words also appear hovering in the same fashion as in the image above.

I cannot reproduce this error on other wikis such as dewiki or frwiki.

Also the text "Search for contributions" in all wikis appears left-aligned, except on eswiki, which appears right-aligned.

Could this be caused by some local CSS/JS?

https://es.wikipedia.org/wiki/Especial:Contribuciones/MarcoAurelio?safemode=1 removes the weird word, but I cannot see any errors in the console.

The text at the right should be where that arrow is, but this is being overridden in the local CSS. So it should be removed from there.

.mw-collapsible-toggle {
    font-weight: normal;
    /* @noflip */
    text-align: right;
    font-size: 90%;
}

Thank you all for detecting the issue. I have messaged @Leoncastro locally so he can advice the interface-administrators which changes are needed.