<Vendor Namespace> - Here namespace of the vendor custom theme or inbuilt theme namespace I.E. Magento/Luma Here luma is vendor namespace
<language package directory> - Here language package directory like en_us or nl_nl or en_gb
<language package description> - Here add description of the package like English Us Package
<language package code> - Here code of the language package I.E en_US or nl_NL or en_GB
After create above files and directories language_package_code.csv will goes to Vendor Namespace directory
Example
/app/i18n/luma/en_us/en_US.csv
or
/app/i18n/luma/en_gb/en_GB.csv
or
/app/i18n/luma/nl_NL/nl_NL.csv