Issue #2953360 by quietone, heddn, yogeshmpawar, phenaproxima, Gábor Hojtsy, maxocub, catch, alexpott, masipila: Experimental migrate_drupal_multilingual module

(cherry picked from commit 20f44c05)

Issue #2912348 by maxocub, Jo Fitzgerald, masipila, heddn, phenaproxima: Handle entity_references related to Drupal 6 and 7 node translations with different IDs

Issue #2908282 by heddn, phenaproxima, Jo Fitzgerald, quietone, xjm, larowlan, mikeryan, dipakmdhrm: Throw exception for source plugins without a source_module property