Of course you would need to know Punjabi, so start by learning the Gurmukhi alphabet and Punjabi vocabulary. I'd recommend using the Mahan Kosh dictionary when trying to understand SGGS ji.
Now it's true words from Sanskrit, Arabic and Persian are drawn from fairly often. But this doesn't necessarily mean you need a mastery of all these languages. Learn the basics I suppose, just enough that you can find and reference the meaning of words from these languages when they show up.
Afterwards learning some Braj Bhasha is also a good idea, since you'll need that to start understanding the Sri Dasam Granth Sahib ji. But start with SGGS ji yes.
I am far, far from a knowledgeable person when it comes to the languages in Gurbani, so can others here please be more proactive here when it comes to giving responses? There are obviously many well educated users here. When someone asks for help, we should do our best to help them out no matter how basic the question.
Waheguru Ji Ka Khalsa Waheguru Ji Ki Fateh