Step 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.

Step 2. Install and activate a Child Theme

You can go and download an 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 downloaded from the link above and click the Install Now button.
After the installation is completed, click on the Activate link

Step 3. Downloading Extra language files from here.

Step 4. Preparing the files

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 log in to the FTP server.
Once the plugin is installed, go to the left menu "File Manager" -> wp-content / themes / ExtraChildTheme-master folder, where you should see two subfolders:

theme - this will contain the Extra general translation
builder - Create the builder folder manually if you 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 the PO Edit app (which can be downloaded from here).

Step 5. Click on Create New Translation Button:

Step 6. Choose the language to which you want to translate Extra:

Make your translations or change the existing ones, then save the file in the format extra-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: extra-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 extra-xx_XX.po and extra-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 translation files related to the Divi Builder interface and elements.

For the Divi Builder, ensure that the files are saved as et_builder-xx_XX.po where the xx_XX represents your country code. For example, for Romanian, the file will be named et_builder-ro_RO.po.

Using your child theme

In case there is already a child theme installed and activated on the server, first, create a sub-folder name lang , and inside it, create new folders: theme and builder. With the correct folder structure created, copy/paste this PHP snippet into functions.php File:

function wpcninja_translation() {
	load_textdomain( 'extra', get_stylesheet_directory() . '/lang/theme/extra-xx_XX.mo' );
	load_textdomain( 'et_builder', get_stylesheet_directory() . '/lang/builder/et_builder-ro_RO.mo' );

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

Make sure that in both folders, the correct files are being placed: