Menu language not properly migrated from D7 localized menu links.

tl;dr:
On localized menu links from a D7 source, the language is not migrated and the default menu language is assigned to all links.

Description:
I added the configuration for migration from a D7 site using migrate-upgrade and drush8:
drush migrate-upgrade --legacy-db-url=mysql://db:db@localhost/db7 --legacy-root=http://localhost:8087 --configure-only

I executed only the menu and menu links migrations (they do not have other dependencies, as long as the menus have absolute paths)

drush ms | grep menu
  Import from Drupal 7 (migrate_drupal_7)   upgrade_menu_settings                             Idle     0       0          0                                 
  Import from Drupal 7 (migrate_drupal_7)   upgrade_d7_menu                                   Idle     12      12         0             2018-11-14 14:54:18 
  Import from Drupal 7 (migrate_drupal_7)   upgrade_d7_menu_links                             Idle     298     298        0             2018-11-14 14:54:50 
  Import from Drupal 7 (migrate_drupal_7)   upgrade_node_translation_menu_links               Idle     298     0          298                               
drush migrate:import upgrade_d7_menu
drush migrate:import upgrade_d7_menu_links

Now I have the menus in my new D8 site, and I also have the links. All of the links however point to the default language.
D7 db entries:

MariaDB [db7]> select language, link_title from menu_links where menu_name like "menu-main%" order by link_title limit 15;                           
+----------+--------------+
| language | link_title   |
+----------+--------------+
| fr       |  Jeunesse    |
| fr       | 2017 Thoune  |
| de       | 2017 Thun    |
| it       | 2017 Thun    |
| fr       | 2018 Zoug    |
| de       | 2018 Zug     |
| it       | 2018 Zugo    |
| it       | 2019 Chillon |
| de       | 2019 Chillon |
| fr       | 2019 Chillon |
| it       | 360°         |
| fr       | 360°         |
| de       | 360°         |
| fr       | A paraître   |
| de       | Aargau       |
+----------+--------------+
15 rows in set (0.00 sec)

D8 db entries:

MariaDB [db8]> select langcode, title from menu_link_content_data where menu_name like "menu-main-navigation" order by title limit 15;               
+----------+--------------+
| langcode | title        |
+----------+--------------+
| de       |  Jeunesse    |
| de       | 2017 Thoune  |
| de       | 2017 Thun    |
| de       | 2017 Thun    |
| de       | 2018 Zoug    |
| de       | 2018 Zug     |
| de       | 2018 Zugo    |
| de       | 2019 Chillon |
| de       | 2019 Chillon |
| de       | 2019 Chillon |
| de       | 360°         |
| de       | 360°         |
| de       | 360°         |
| de       | A paraître   |
| de       | Aargau       |
+----------+--------------+
15 rows in set (0.01 sec)

One would expect that the language should be migrated too.
It is possible to change the language from the DrupalUI afterwards, so Drupal 8 is configured correctly to handle multilingual menus.

I mark this as major, since it makes the migration not usable from/to multilingual sites. Please feel free to change.