Internal is sometimes your only option early on if you're not close to anyone, but that's okay - you can get your cities up and running quickly. Sometimes it is useful to have an internal trade route or two going, particularly if you've just founded a new city - trade routes have really made getting new cities going much easier.
That being said, international trade is a major (usually the main) source of gold, so you can't neglect it. And certain civs have bonuses that make international trade routes far more profitable than internal trade routes. Portugal's UA makes it very difficult to justify internal trade routes (I only used one, along with a purchased aqueduct, to get a city on another continent up to 9 pop almost instantly; then I switched it back to international). They get so much money from international trade it's really crazy. I imagine only Venice beats them in this department.
Morocco is another civ which really incentivizes you to go international as opposed to domestic - you get bonus gold and culture for each different civ you're trading with. However, there's one caveat to this - Morocco's UA incentivizes other civs to trade with it, meaning they get huge bonuses from international trade even without committing their own units! Meaning they can spare a few internal trade routes. I've got an Epic Portugal game still running (been playing this one a few days now) and virtually all of my most profitable potential trade routes are with Morocco. They were useless at first because I started so far away from their capital (right beside a desert, which they haven't used AT ALL lol), but as soon as they set up a couple of port cities it was time to trade.
Arabia also benefits more from international trade than other civs (if they have a religion). Arabia could really dominate the religion game now - and have a healthy economy as a sweet side benefit.
As for Venice, you need to focus mostly on gold since that's Venice's lifeblood, but since they have so many trade routes by end-game, it can't hurt to have a couple of them supplying Venice with food and production.