You can set up the different default languages for your editable elements (instances) on the same page using the init() WProofreader initialization in a bundle with the setLang method and syncOptions parameter. To do so, please follow the next steps:
1) Initialize WProofreader using the init() method when you clearly know in which control you want to initialize WProofreader which provides additional management options.
2) Pass the appropriate language shortcode for the setLang in WEBSPELLCHECKER.init, for example, using it in the combination with getInstances() method.
WEBSPELLCHECKER.getInstances()[0].setLang('NEW_LANGUAGE_CODE')
3) Disable synchronizing ignore options and languages between all the created WProofreader instances using the syncOptions in WEBSPELLCHECKER_CONFIG.
syncOptions: false,
4) Also, it would be great to check and, if needed, disable storing of the language between browser sessions using disableOptionsStorage in WEBSPELLCHECKER_CONFIG.
disableOptionsStorage: [lang],
