The Files Used For Translating

You’ll have one pair of PO/MO files per language and region, but only one (optional) POT per domain.

PO (Portable Object) file

A list of readable “translated objects”

MO (Machine Object) file

The corresponding binary (to be interpreted by Gettext when doing localization).

POT (PO Template) file

Contains all existing keys from your source files, and can be used as a guide to generate and update all PO files.

Not mandatory. Depending on the tool you’re using to do l10n, you’ll be just fine with only PO/MO files

Creating custom language files for a child theme

A child theme always uses the translation of the parent theme. You have to use a different / the child themes text-domain slug for all translation function calls in the child theme files

Copy the .po and .mo files to: “wp-content/languages/themes”

Name them “[yourchildthemename]-nl_NL.po” and “[yourchildthemename]-nl_NL.mo”

Replacing [yourchildthemename] with the name of your child theme.