Hello,
Most messages can be found in <tomcat home>/webapps/alfresco/WEB-INF/classes/alfresco/messages
For example the "inline-edit" one can be found in <tomcat home>/webapps/alfresco/WEB-INF/classes/alfresco/messages/slingshot.properties
Then you have 2 solutions:
1- Dirty
If you directly replace in the file, it should work but it's dirty. It's not grouped with other modifications and will be lost with an alfresco update.
2- Override it
In these files you can retrieve the IDs of the messages you wanna override (e.g.. actions.document.inline-edit).
Then you can create a new language file overriding this message (try googling it a bit, if you don't manage to do it come back here we'll help).
HINT: it includes creating both a spring bean to register the language bundle and creating the language files.