I had my share of language lessons. It started in middle school with Spanish, then English in high-school, then Mandarin and Italian for a year each. I also delved into Macedonian, Romanian, Polish, and German, to little effect. French and English remain the only two tongues I practice on a regular basis, but I want to broaden my horizon.
Spanish would be the most logical choice since I've been studying it for 8 years. It's close to my mother tongue. Even though I lack the vocabulary to express myself, I understand most texts and conversations in Spanish. I could also learn my grandparents' tongue, Vietnamese.
There is only one language I miss talking though, and that is Italian.
When it comes to learning a new tongue, my strategy has always been the same. I open Duolinguo, write down the words and expressions I come across, and memorize everything with flashcards. I then complement my notes with online resources.
Daily writing could also play a key part. Visualization plays a huge role in the memorization process, so staging the words you come across in a small text could definitely leverage that. A mental palace on paper to learn a language.