I'm not really sure what you want to do... do you want to shoot videos with a camera and then edit them (i.e. cut them into a movie of our liking)? Or do you want to take an already edited movie and then add lower thirds, scrolling titles and such things? To me that sounds more like "Video Creation"...
If so, you might want to take a look at Apple "Motion". At a very reasonable price, you get some fine options for banners, text animation and nice effects.
If you want to actually edit a movie, then cgbier is right about "Final Cut Pro X", it can do anything you want. But for titles and lower thirds, you might still want to add Motion to that.
As for DVD authoring: It is complicated.

I have yet to find an authoring tool for Mac that I am totally content with. And if you want multilanguage, you will have to pay a surprisingly big amount of money to get something usable. I can not fullheartedly recommend any of the software I have tried. So I would actually try to have people deal with files instead of DVDs. Multiple audio tracks are supported by virtually any encoding software (e.g. "Compressor" or even the free "Handbrake"), and nowadays most DVD players can play them just as easily.
A while ago, Steve Jobs has announced that he declares the disc to be dead and Apple has not offered an upgrade on DVD authoring software since. First I was really shocked, seeing that I love to buy DVDs, for the extras alone and because of the lack of original language video-on-demand in Germany. But by now, at least for my personal amateur videos, I have to kind of agree with him. Many of my friends don't live next door anymore and some of them even don't have a DVD player. The easiest way for me to share my videos with them is to send them a link...