Using different translation of HTML email template files
By default sites based on WordPress are single-language. The WordPress Multilingual plugin (WPML) makes the site multilingual. In this case you can translate email templates.
To use different translations of HTML email template files, you need to create a directory for each language.
For example, if you want to translate to Thai, you will need to create subfolder /th_TH/ in the directory /wp-content/themes/{YOUR_ACTIVE_THEME}/ultimate-member/templates/email/ and then copy English email templates from the directory /wp-content/plugins/ultimate-member/templates/email/ to the /wp-content/themes/{YOUR_ACTIVE_THEME}/ultimate-member/templates/email/th_TH/
Once files are copied you can edit these new files and translate the English to Thai.
See also Email Templates, How to translate plugin.