A user's default language is set in the My Account settings for the user. See My Account - Preferences - Settings.
If an administrator creates an object, the default language for that object is that administrator's default language at the time the object is created.
All other administrators view the metadata and editable fields for the object in their own language, if available. If the data is not available in their language, they view the data in the object's default language. For example, if a custom field is created by English administrator, a Spanish administrator views the field in English if a Spanish translation does not exist.
If an administrator edits an object, all of the editable fields shown are copied to that administrator's translation, even if the translations did not exist previously. For example, for a field created by an English administrator, a Spanish administrator automatically edits the Spanish translation. If there was previously no Spanish translation, they see the English translation, but their edits are saved as the Spanish translation.
For a required field, the default language translation must be populated. It cannot be empty.
If a localizable field is required and does not have a translation for a user's language, the user views the field in the default language of the administrator who last saved the field. For example, if a Spanish translation is not available for Task Name and the administrator who last saved the field is an English user, a Spanish user sees the English translation for Task Name. In this case, English is the default translation.
If a localizable field is not required and does not have a translation for a user's language, the field is populated with the default language translation.