polymulti

While I'm not Sprachprofi, I have some knowledge of more than a dozen languages (and speak 4 fluently). Learning more languages is always helpful: if you travel, it doesn't matter if you speak 20 languages if none of them are spoken there. If you want to read literature in the original language, you need to learn that language, no matter how many or how few you speak. If you want to bring a smile to someone's face, speaking to them in their language often does it; they don't care if you speak 2 languages or 50. If you want the fun of exploring a new grammar, it doesn't matter how many you already know.

Within polyglot circles, I hear a lot more discussion about quality of knowledge and the value of enthusiasm, as opposed to sheer numbers of languages. The latter just tends to be easier to summarize, and catch external attention more often, I'd say.

Languages have an almost infinite number of purposes, and a wide range of abilities are useful in a wide range of circumstances. Only you can answer the question of how many it makes sense for you to know, and to what level - and your answer will not apply to everyone else.

Yes. English has gone global - it's spoken by a lot of people on every continent - but most people do not speak even very basic English. It's still useful, though.

Esperanto is also global, but has around 0.1% as many speakers as English. I've run into Esperanto speakers randomly, and it's been incredibly useful to me as I travel. It's differently useful from English: there's a community of speakers, and I've met some very kind, helpful people through it, and couchsurfed with some of them. On the other hand, for asking directions, I usually use English or a local language.

Having a language everybody spoke would be quite useful. I don't think that will happen within our lifetimes, though.

It is: and it's also amazing how much it hasn't. I've had to rely on broken, basic Russian in Hungary when English wasn't an option, and a friend of mine used Serbian in similar circumstances. I had to use a combination of a Czech-Esperanto dictionary and broken Dutch at a falafal cart near the Slovakian border once. I've had to translate Italian<->English for confused Italian toursts who speak no English and have traveled to the UK. I've translated English<->Italian for some Chinese tourists who spoke English but no Italian and who wanted to order food from some Arabic speakers who'd learned Italian as a foreign language but spoke no English - I was in line behind the tourists. I've done German<->Italian in Germany occasionally for Italian tourists, too. I'm not in tourism; I'm just a traveler who helps out when people are clearly struggling.

http://www.britishcouncil.org/learning-faq-the-english-language.htm claims that 3/4 of people speak no English whatsoever. Of the 1/4 who speak some, a lot are at a very basic level, where they can manage some touristy things but not hold conversations or understand even advertisements, much less popular culture.

http://en.wikipedia.org/wiki/English_language_in_Europe claims 51% of Europeans speak at least some English - so, even in Europe, half of people don't. People sometimes joke that the language of Europe is broken English; it's odd to watch two people speaking it non-natively and understanding each other, while a native speaker is confused and can't understand them.

Couples tend to have a common language. Sometimes it's English, sometimes it's the native language(s) of one or both of them, sometimes it's the language of a country where they're both studying or working, etc. I know couples who have Esperanto as their only common language, too. Couples without a common language tend to get one, or split up.

Go for "Homaranoj" if you want to be really Esperanto-y about it ("ho-mar-AN-oy"). :)

It's not just "near impossible". The languages have different word order, different ways of expressing things, different parts that need to agree with each other, etc.

Do you 'have' or 'take' a shower? It depends on the language. Does the "blue" in "blue flowers" need to have a certain gender and/or be plural? How do you handle that if you're putting flowers in French and blue in Chinese or English, which don't do that? Etc.