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.
Warning: We highly recommend to use child-theme for public free or pro themes. Do not edit template files within the core plugin itself, as they are overwritten during the upgrade process and any customizations will be lost. We recommend using a child-theme, which has no dependencies with the official themes repo, so your custom files cannot be rewritten after a theme upgrade.
See also Email Templates, How to translate plugin.