How do I make multilangual content synchronization between two sites?
On the source site:

Make sure that your view has a column that contains an unique identifier for the item. This value must be the same for every language of the item.
Add a column to the view that contains the language code. For English this would be 'en' and for French this would be 'fr'.

On the target site:

On the feed type, map the unique identifier either to:

A non-translatable field;
Feeds item: GUID.

And mark that as unique on the feed type.

Map to all the fields that you want to map to.

Map the langcode value to the &quot;Language&quot; target.


Known issues
Note that there are some known issues with importing multilingual content using Feeds. I'm doing my best to resolve them.
You can currently run into the following errors:

this field cannot hold more than 1 values.
Patch available at https://www.drupal.org/project/feeds/issues/3155046

A translation already exists for the specified language
https://www.drupal.org/project/feeds/issues/3126351
No patch yet, but you could help by confirming or correcting the steps to reproduce the issue.

Column cannot be null (SQL error)
https://www.drupal.org/project/feeds/issues/3169398
You can help by describing the exact steps to reproduce this issue.