According to the US Foreign Service Institute, which trains US diplomats, there are 10 ‘category 1’ languages that share the most similarities with English, and are hence the easiest to learn. These include French, Spanish and Italian, and they all take about 600 hours of study to become reasonably fluent.
If you want to really impress, though, why not try Afrikaans? This is also category 1 and is more unusual as a second language. Alternatively, you could impress your partner with your earning potential and learn a programming language. Ruby is one of the easiest languages for novice coders and one of the highest paid. Programmers fluent in Ruby typically earn £79,000.