Multilanguage CSV Import possible?

Tagging issue with "multilanguage", so I can find this issue back as soon as I plan to focus on Feeds multilingual issues again.

@MegaChriz: Just wanted to add that I am also very interested in importing content to different languages/translation sets

After many trials, I have found a way to import multilingual content by Feeds. Since it appeared to me pretty complex to achieve, I share this with everyone looking for a solution. I imported multilingual content between two Drupal sites via JSON (Feeds extensible parser and JSON API Include modules), but I suppose it should work for other use cases as well.

As suggested in #3 above under 1, I could achieve multilingual import in two steps, by creating 2 different feed types (eg., feed_type_se (default language, sweedish), feed_type_en (the translated language)).

1. For feed_type_se (default language) I configure the feed type as follows:
- Language: Sweedish
- Insert new content items
- Update existing content items

When creating the feed, I provide the feed source in the default language, something like this:
https://sourcesite.se/se/jsonapi/node/article?jsonapi_include=1&filter%5Bfield_issue_reference.id%5D=a7d6789c-a2af-4ee7-819a-8206b9ce25e2
(I want to import a list of articles belonging to a journal issue, identified by the uuid a7d6789c-a2af-4ee7-819a-8206b9ce25e2)

2. For importing the translated content, I used this configuration while creating the feed type feed_type_en.
- Language: English
- Do not insert new content items (!)
- Update existing content items

Configure the map so that the translated items are mapped to the translated target fields, like source "title" is mapped to "title" in English, etc.

When creating the second feed, I provide the feed source in the translated language (here English), something like this:
https://sourcesite.se/en/jsonapi/node/article?jsonapi_include=1&filter%5Bfield_issue_reference.id%5D=a7d6789c-a2af-4ee7-819a-8206b9ce25e2

As for the unique field for the import to be mapped, I created a custom JSON ID field (field_json_id) on the target content type to be fed from the source. The source that will be imported into this field is the UUID as above (in JSON this appears as id of the source site item).

The tricky part is that this field_json_id, which is used as the unique field has to be marked as a translatable field (Users may translate this field) on the content type field setting. (Otherwise you get the "this title already exists" error message). And you should choose the translation language for the unique field as well, as in the image below.