How can i insert term in a specific language of Polylang?

You can actually make it this way:

$cat1 = wp_insert_term(
  'Cat1',
  'category',
  ['slug' => 'slug-cat1'],
);

$agenda = wp_insert_term(
  'Agenda',
  'category',
  [
    'slug' => 'slug-agenda',
    'parent'=> term_exists( 'Cat1', 'category' )['term_id'],
  ],
);

pll_set_term_language($cat1['term_id'], 'fr');
pll_set_term_language($agenda['term_id'], 'de');

Here is link to Polylang Function reference:
https://polylang.wordpress.com/documentation/documentation-for-developers/functions-reference/