Taxonomy terms are fetched with "get_term_by" during taxonomy import -> localisation is not supported.

[ironland]

We have to change the method to get_terms which has support for language through polylang. Here's a quick example:

$lang     = ! empty( $this->i18n['locale'] ) ? $this->i18n['locale'] : \pll_default_language();

$found_terms = \get_terms( [
    'slug'       => $slug,
    'taxonomy'   => $taxonomy,
    'lang'       => $lang,
    'hide_empty' => false,
    'fields'     => 'ids',
] );

the existence of pll should also be checked in the code.