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Overview of the issue
If you are using BuddyBoss plugin, Member Activity Feeds cannot be translated.
Workaround
Open wp-content/plugins/buddyboss-platform/bp-core/compatibility/class-bb-wpml-helpers.php and comment out the line 83:
//add_filter( 'bp_init', array( $this, 'bb_prevent_translated_post_activities' ) );
However, all previous activities in secondary languages are not registered. It will work for new activities only.