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.