In the olden days, in order to create a custom language pack, people had to manually copy over all the required text files, look up the text to be translated in the UI to understand the context, and zip all the files back.
Atlassian has since provided a more streamlined way of creating translations, allowing you to create translations directly from the JIRA UI.
For this recipe, we need to have the InProduct translation for the JIRA add-on installed. You can download it from the following link and install it with the Universal Plugin Manager:
https://marketplace.atlassian.com/plugins/com.atlassian.translations.jira.inproduct