<?php

// …

function l10nFilterQuery($query)
{
    $langs = [
        'fr_FR',
        'en_UK',
    ];

	// Stop if the query is only for a category.
	if (1 === sizeof($query->query) && isset($query->query['category_name'])) {
        return;
    }

	foreach ($langs as $currentLanguage) {
        if (USER_LANG !== $currentLanguage) {
            $excludedCategory = get_category_by_slug(
                strtolower($currentLanguage)
            );
            if ( false !== $excludedCategory ) {
                $excludedCategoryId = $excludedCategory->term_id;
                $query->query_vars['cat'] .= '-' . $excludedCategoryId;
            }
        }
	}
}
add_action('pre_get_posts', 'l10nFilterQuery', 10, 1);