How can I drive these Language.php changes into my Wiki (running 1.4.0)?
There's a script you can run, maintenance/rebuildMessages.php or the like. But the best way would be to go and find the message you want to change on the Special:AllMessages page, then follow the link and change it there in the wiki database as is.
When you install MediaWiki, it copies the default Language.php settings into the database and ignores that Language.php file afterwards (except in 1.4.x there is an switch in DefaultSettings.php that tells it to just use the file and not the database).
Make sure you're adding settings to LocalSettings.php and not changing the DefaultSettings.php file. :)
HÃnandil