When adding observation links, Twitter can’t access the data required when using links with ?locale=en to specify correct language version (default is english regardless of account settings). Example:
Twitter Card Validator https://cards-dev.twitter.com/validator says: “ERROR: Fetching the page failed because it’s denied by robots.txt.”
Looking html the pages look similar, correct meta tags are available.
With this problem only direct links to language version are possible (twitter doesn’t add the card, just plain url), or using default link that directs to english page and shows english names in Twitter card.