Use WordPress native translation

You’re right. Anywhere that a string exists in the plugin that already exists in core it should use default as the text domain instead of customize-posts. Is this something you’d want to open a pull request to change?

Here are the strings in Customize Posts via php tools/i18n/makepot.php wp-plugin src/wp-content/plugins/customize-posts/: https://gist.github.com/westonruter/08ca016d4e2ada375dc2f26895d9f243

@viewup if you would like to contribute, you can follow up to amend the PR that I opened here: https://github.com/xwp/wp-customize-posts/pull/364

I use the plugin most times on work, so is hard to contribute without permission. But i will try to help outside.