Not all messages are accessible by < F12>. There are many system messages that popup. These can be translated by using TOOLS – TRANSLATE MESSAGES.

Note here you cannot translate Default Translations so choose your own Translation Collection and work your way through the list translating the messages.  You can filter the list by entering search criteria into the blue filter row at the top of the list - in the example above, a filter of "Fea" has been entered to find all records starting with this text.  Click on the "A" icon to make other search filters and click on the clear filter icon to clear a filter .  All items on the Trees start with a Key of "TREE". 

You can only edit the text in the Translation and Comment columns.  The Comment field contains information to explain the context of each message. 

Click on the Apply button to apply the changes.  You will need to exit from CMSi to see these changes.