This may be confusing as it may seem the default comes from the system parameters form. However on importing users these are actually defaulted from the importing Admin user.
Scenario: The System Parameters form has language set to EN-GB, the default company language is also set to EN-GB but when a new user is imported their default language in User Options is set to EN-US. To address this problem identify the User Options for the logged in user who imported the new user or the Admin account use for the integrating application, if the user is being created through an integration.