English is my mother tongue thanks to the Normans invading my country around 1190. I'm also able to speak Irish fluently, because my grandparents used to drill the language into me when I was younger and because Irish is compulsary in my country's education system until you leave secondary school. Sadly, due to the fact that my primary school teachers were awful and because the Celts couldn't come up with an independent alphabet (Ogham was destroyed by Latin characters and because the Celts didn't write at all, the Normans got to determine how Irish would be written), I'm awful when it comes to reading or writing Irish. Protip: never let your language use another's alphabet. Unless you're French, your language is going to be incredibly hard to understand.
On the other hand, I'm able to write and read French easily, and I know a lot of vocabulary, but when it comes to talking... ugh, it's horrible. I can never get those nasal sounds right and when I find myself in the rare circumstance that I'm talking to someone French, their accent is normally to thick to comprehend anything.
Learning German, Japanese or Latin would be ideal, as well. Seeing as the Germans are going to walk away with an amazing economy, I'd probably be able to get a job over there. Japanese would be cool because working in the technology capital of the world would be pretty awesome. And Latin impresses everyone, right? I'll probably never learn any of them though. Don't have the time, or the willpower.