The customizer UI automatically takes the language but you may need to change the translation to a specific language.
How to do
You can change the translation of the text on Clipart Galleries and Messages and alert.
go in your Zakeke back-office and first of all choose the specific language you want to work on (see below):
Go on tools > Images and Clipart Galleries > click on the edit button to change the text in the language to the desired one.
Messages and alert
Go on settings > Messages and alert > click on the edit button to change the text in the language to the desired one.