1. Setting up WordPress to use your language

You can go to your Dashboard (left menu) > Settings > General > Site Language.

Choose the language from the dropdown menu and click Save Changes.

2. Install a Child Theme

You can go and download a Extra Child Theme from here.

Follow these steps to install & activate it:

Go to Appearance -> Themes -> Add New -> Upload Theme
Choose the zip file, you have downloaded from the link above and then click on the Install Now button.
After the installation was completed, click on the Activate link

3. Downloading Extra language files from here.

4. Preparing the files

First install a 3rd party plugin called WP File Manager. This will help you to download and upload files directly from your dashboard without the need to login to FTP server.

Once the plugin is installed, go to the left menu "File Manager" -> wp-content / themes / ExtraChildTheme-master folder, where you should see one subfolder:

theme - this will contain the Extra general translation
builder - Create the builder folder manually folder in case you also want to edit the Divi Builder strings.

Once the zip file was downloaded at step 3, locate the folder called lang, open the .po file related to your language using PO Edit app (which can be downloaded from here).

Click on Create New Translation Button:

Choose the language to which you want to translate Extra:

Make your translations or change the existing ones and then save the file in the format of xx_XX.po where xx indicates the country code.

If you're not sure about the country code, you can check on this page your language and, on the 3rd line it should have the country code:

Example: ro_RO.po is the file name for the Romanian translation.

The saving process will create a new file with the extension of .mo .

Using the File Manager plugin, upload both files xx_XX.po and xx_XX.mo to wp-content/themes/ExtraChildTheme-master/lang/theme folder.

Repeat those steps for the .po file located in the zip file downloaded at step 3 includes/builder/languages . Those are the translations files related to Divi Builder interface and elements.

Using your own child theme

In case you are using your own child theme, you need to follow up the same folder structure as presented in the example child theme and also put this PHP code into your functions.php file from your child theme:

function wpcninja_translation() {
	load_child_theme_textdomain( 'extra', get_stylesheet_directory() . '/lang/theme/' );
	load_child_theme_textdomain( 'et_builder', get_stylesheet_directory() . '/lang/builder/' );

}
add_action( 'after_setup_theme', 'wpcninja_translation' );

Note: make sure that in case of using your own child theme, in your child theme's folder, you create a folder called lang . Inside the lang folder create 2 sub-folders:

theme
builder

Place your translation into those two respective folders.