To translate Magento2 multi-seller module to Spanish from English you can change the module keywords.
You need to follow this path in the module unzipped file app/code/Webkul/Marketplace/i18n.
Here you can find a CSV file with the name “en_US.csv”. Now you need to rename the CSV as “es-ES.csv”.
Now you need to open the CSV file and translate all the keywords mentioned under the file to Spanish.
After editing the CSV file you can upload it to the servers where you have installed the Magent2 Marketplace. The module will get translated into the Spanish Language.