Front page on a multilingual site intermittently displays incorrect language

I have a multilingual (English is the default language) Drupal 7 site. The language detection is set to URL, with the domain being the part of the url that detects language.

Every now and then (approximately every few days) the front page, and the front page only, will display the default language (English) as opposed to the correct language.

I believe it is related to the page cache, as clearing the cache rectifies the problem, but I cannot work out what is causing the problem.

Drupal version: