How to make a content visible only in the corresponding language it has been created on a multilingual website
Entity Translations can be configured so that when it doesn't find content for a field in a language, it uses the content of the default language if any is available. If you're in an English page and a field is missing content, it may pick up the French content.

This can be disabled in /admin/config/regional/entity_translation by unchecking the "Enable language fallback" checkbox. When disabled, fields that have no content will simply not appear and won't fall back to the default language's content.