How to fix the error "Warning some translations files are missing for that ISO code" in PrestaShop
This error usually happens when you import a new language and you use a theme other than the default prestashop theme. In case you use another theme, after you install a new language you'll see this error when you go to the Languages sub-tab of your backend's Tools tab. On the Languages sub-tab page there's a table with all the languages you have installed. The new language will be listed there but under the column Enabled there will be a warning yellow triangle. When you hover the mouse pointer over it you'll be shown the message that some translations files are missing. Actually, right after you import a new language from the Translations sub-tab of the Tools tab (and your theme is other than the prepackaged prestashop theme), you may get the error message: Translations added successfully but the language has not been created.
Both errors are fixed in the same way. You need to copy one file from the prestashop theme to the one you're using. If your PrestaShop is installed directly in the root public_html directory on your PrestaShop hosting account, you need to go to public_html/themes/prestashop/lang. In that lang folder you'll find the PHP files corresponding to the languages that you have installed. For example, if the language in question is French the file would be labeled fr.php. Copy the file for the particular language and paste it in the lang folder of your current theme (e.g. public_html/themes/current_theme/lang). You can copy the file from the Files section of the Pixie control panel, or you can use an FTP client (e.g. FileZilla). Then refresh the backend of your store and everything should be fine.