All strings, even time units, are customizable. If you place a strings.yml file in the plugin folder, it will take priority over built-in languages. If you want to change strings, look at PermissionSigns/src/main/resources/strings.yml for the proper keys. All strings have the format: ENUM: "Displayed string". The enum must be identical as it defines which string you have changed. All strings belonging to a language are beneath the language code and indented with two spaces.
The easiest way to add a new language is to copy an existing language and paste it into your custom strings.yml and change strings as necessary. If you don't include all strings, the remaining will use the built-in English translation. Remember to change the language code to whichever you use for your custom language.