Actually, I think you've touched on two very important things about language learning.
The first one being pretty straightforward- incentive. That is probably the biggest factor in language learning. For a baby, learning their L1 could possibly mean life or death... or at least communicating very important things to your parents, who are usually 100% of the people you know (unless you have siblings).
Finding an incentive in second language learning is an extremely difficult area. If you grow up in a small European country smack dab in the middle of the continent, I would imagine there would be a lot of incentive to learn many languages. If you are someone who travels a lot on business, there would be an incentive. If you are immersed into a language, which can often mean living in one of that language's countries, then you also have incentive. Finding incentive while studying from a book is very hard for some people (unfortunately myself included).
The second thing you brought up is the idea of sign language being taught to all deaf people. Not to take the focus away too far from what you originally said, but there is a huge degree of language that doesn't include words, and that is a big part of language acquisition. Drawing pictures to get your point across is even good. Teachers and schools seem to avoid this aspect of communication, which is unfortunate because that's usually what communication will boil down to for some people. There are a lot of things about a language that are specific to that language (or at least different from some other languages). Like for example, if you see a kid drawing a picture of the sun in America it will most likely be yellow. In Japan, however, the kid will most certainly color it red. It's hard to say just how heavily that impacts language, but I believe that is a huge part of it. General body language and really watching/looking at the person you are talking to will give hints that spoken language will not always convey. Both the way you look and how you act, however, are bound to your languages rules.
I feel a little bit rude bringing this up, but I feel like it connects with the topic at hand. I was talking to some friends recently (who are in their 50's - 60's) and they were talking about how lately when Japanese go on trips to Asian countries, like Guam and places like that, they try to speak English to the people to communicate what they want. Unfortunately something hasn't been working out communication wise recently to the extent that people of those countries have been learning simple Japanese phrases in order to deal with the Japanese. I think it's interesting to note that a lot of practical English use will happen with non-native English speakers for Japanese. I've seen it happen in hotels and other places like that- a Japanese person speaking English and maybe a French person speaking English. Those conversations usually take a long time, but usually seem to work out. I still can't get over the idea of a Person from Guam speaking in Japanese to a Japanese person who is speaking in English. That would be a trip to watch
