Placeholder tokens not translated

I have tried to set up a Trigger to display the following welcome message to a user

Welcome [user]. You last logged in on [log-date].

But the tokens are not translated into the actual user name and login date. They just appear as above. However if I substitute user token [user] with the global token [user-name] that IS translated. I've trawled for documentation on this but am still struggling with understanding this useful module.

Any help would be appreciated