Moodle provides a neat tool for admins to customise the strings in the installed language packs. This is the language customisation tool. It is available from:
site admin -> language
This is very useful, because you can change strings all over Moodle without touching core code, and the changes are all saved in the database. So they survive a backup,restore,Moodle version upgrade, or server migration. This allows you to customize text across Moodle without needing to write any code at all.
By default Poodll does not offer the ability to change the language strings that you see in Poodll. But using Moodle's great toolset you can do this.
First, open the language customization tool ..
Site Administration -> General -> Language -> Language customization
Then choose the language pack to edit. This will be the language pack that your site or course is using.
Open the language pack for editing
Select the Moodle plugin(s) and search for the line(s) of text you wish to change. Once you have found the text you will be able to add an alternate version and save it to the language pack.
A simple way to do this is to just select the plugin that you will edit and list all the strings. (ie do not enter any text to search for). Then find the strings to edit from the list and edit them.
Then add your customizations in the local customizations area. When finished all the strings, you should use the "Save changes to language pack" button to save them.
Finding the String indentifier
If you are not sure if the plugin strings can be customized, or you want to find the string identifier to make customizing faster, Moodle has feature to help you.
First turn on the display of string identifiers at:
Site administration -> Development -> Debugging
Then in the URL of a Moodle page append &strings=1
Then Moodle will print the string identifier after each string. The display will only change for you, other users will not see the identifiers.
More details on the tool are here:
https://docs.moodle.org/en/Language_customisation