Importing 'alt' and 'title' fields does not work for custom image migration
You're only mapping the fid there - you need to also map the alt and title for them to come over:

field_galerie:
  plugin: iterator
  source: field_galerie
  process:
    target_id:
      plugin: migration
      migration: luxus_files
      source: fid
    alt: alt
    title: title