Which language should I learn?

I want to learn a new language, and I was considering Scottish Gaelic (Since I grew up speaking Irish). I can understand Scottish Gaelic, to a point, so the languages are similar, and it would be easy for me to learn. But then I got to thinking, and I started to like the Idea of Spanish. But I also want to learn Arabic, or farsi. So ultimately, what language would be the best to learn? Scottish Gaelic, Spanish, Arabic, or Farsi?