Which languages do you speak?: The only language I speak fluently is English.
Which are you learning, or have you learnt (in school, for example)? I studied Latin and Spanish for several years, and would say that I'm moderately comfortable with them. I've tried to learn a bit of Italian and Russian on my own, too.
Which would you want to learn? Any other language I don't know yet. I'm sad that I'm monolingual, but my language skills have been getting worse, not better, and I'm not sure when I'm likely to have time to gain fluency in another language. From a practical perspective, it would be really useful to improve my Spanish from intermediate to fluency.
Does anyone else in your family speak another language? Nope. They studied other languages in school (French, Latin, and German), but don't really remember anything.
If monolingual, do you find that only speaking one language limits your employment opportunities? To a degree, yes, though I don't think it will be a major barrier. I have encountered some positions that want people who are bilingual (often in English and Mandarin, which I'm unlikely to pick up with any degree of fluency at this point). In general, it's not a huge deal, as I'm not in a career field that requires multilingualism, but there are definitely times where knowing another language would be useful. I should improve my Spanish.
If monolingual, what difficulties have you faced in life and online knowing only one language? Online: None, really. I can use online translators to guess at things if I need to, and most of the websites I visit are English sites. I can also read a number of Romance languages, so I can read some of those pages as well. In real life: I've had some situations where I can't communicate well with people. I've also felt left out when people I know have started speaking another language (generally an Asian language) and I can't follow along.
If monolingual, what are your reasons for being so? I've never had the opportunity to gain sufficient exposure to a foreign language to become fluent. Living in an English-speaking area, there has never been a reason to use a language other than English in most of my conversations. I haven't had the opportunity to live abroad, so I haven't been able to pick up a language that way.
If monolingual, which language do you believe would be the hardest to learn, and why? Maybe Mandarin? I've always heard that Mandarin and English are two of the hardest languages to master. As an English speaker, the importance of intonation in Mandarin seems difficult, and I've also never learned a language that doesn't use an alphabet before.
If monolingual, have you ever encountered a language barrier, and how did it make you feel? When I went to Italy, I didn't speak much Italian (I had tried to teach myself a bit before traveling, but had never taken a course or spoken to anyone who knew the language before). While most people in Rome spoke English, those elsewhere did not necessarily understand my native language, and I had trouble communicating. Fortunately, someone else I knew spoke Italian, so it wasn't a huge issue. I've also spent time elsewhere around people who didn't speak English, and communication was difficult.