We’re seeing the same problem in retrieve_translation_details() of vendor/yoast/i18n-module.php.
The function does not properly sanitize the data structure and produces a notice when parsing “Klingon”:
{
"id": "918",
"name": "Klingon",
"slug": "default",
"project_id": "1",
"locale": "tlh",
"current_count": "4",
"untranslated_count": 829,
"waiting_count": "4",
"fuzzy_count": 0,
"percent_translated": 0,
"last_modified": "2015-08-06 16:05:46"
}