Uniconta uses the word 'Default' for language settings slightly differently that you might be used to in other systems.
If the user has selected 'Default' as the language on a customer/vendor card then it is the country on the card that determines the language on the documents (and not the company language selected in Company/Company Information.
For example, if 'Default' is selected on the customer/vendor dard and the country on the card is 'Sweden' then the documents will be printed in Swedish.
NOTE: If you are using Report Generator for your documents, you will also need to check those settings for language.