In order to translate the Magento 2 email template, you have to create a new translated on the basis of the existing one. So, navigate to Marketing > Communications > Emails Templates and press the Add New Template button.

Magento 2 new template adding

First of all, you have to Load Default Template you want to create the translation for. (we have chosen the New Order email template as an example)

Magento 2 email default template

Template Information

Once you've loaded the Magento 2 email template you want to translate, you need to fill such fields as Template Name and Template Subject.

Magento 2 new email template information

Then, translate the Template Content and add the Template Styles if you want and Save the Template.

Magento 2 new email template content and styles

After the saving, you will see your template in the list of all Magento 2 templates.

Magento 2 new template

 Enable Template 

After that, you have to assign a newly created Magento email template in the store configurations. In order to do it navigate to Stores >  Configuration > Sales > Sales Emails (your path can be different depending on the template). Here you change the Store View to the one you've made a translated email template for.

Magento 2 email template configuration 

Once you've chosen the store view you want the Magento 2 email template to be sent from, set the translated template in the New Order Confirmation Template by removing the point from the checkbox and Save Config. (Confirmation Template Name differs in accordance to the template you have chosen)

Magento 2 new template stores configuration

Email Template Translation is just one of the series of articles about How to Configure Multi-Language Magento Website? which include information about the creation of a new store view, language packs installation, useful multi-language store extensions, product&category, CMS pages&blocks, and other translations in Magento 2 website. You should definitely get acquainted with them in order to know how to manage a multilanguage store.