Respnsive design from Google...

!rant
I'll be the first to admit that my web dev skills are average. Where I work we don't get pushed much since we work off of a very limited (custom to us) platform, which doesn't leave one very well rounded. I am very good with CSS/HTML/FLEXBOX/RESPONSIVE DESIGN and have minimal skills in PHP and JAVASCRIPT. In school I've dabbled with SQL, but hell if I remember much (beside key, the whole key and nothing but the key). What other languages or codes do you recommend learning; what's hot right now? Or should I focus on putting higher emphasis on the ones mentioned above first? I am a front end developer and don't plan on ever doing back end so not looking for the toughest of the tough - just something that will help me grow and land a better job. If there's any cool learning sites you know of too, or other tools, please let me know.