Updated a WP plugin and translation needed fixation. I edited the .po file, but no change was on the site. I needed to convert it to .mo file.
Thanks to Joss Crowcroft and his post http://www.josscrowcroft.com/2011/code/php-mo-convert-gettext-po-file-to-binary-mo-file-php/ I was able to convert.
$ msgfmt -cv -o wp-content/plugins/woocommerce/i18n/languages/woocommerce-sk_SK.mo wp-content/plugins/woocommerce/i18n/languages/woocommerce-sk_SK.po 1939 translated messages, 2 untranslated messages.