I have studied Japanese on and off my whole life. When I was a kid I traveled to Japan to visit some relatives and was able to do a lot of simple things (I was 8) and learn some kana. The past few years I have spent more time studying the language and have been able to leverage my Chinese skills to help accelerate my learning.
The thing is I haven’t spent gobs of time studying Japanese, but I can cheat and use my Chinese to get by when I read and watch Japanese media.
I have mentioned previously that I speak Chinese. I have spent a considerable time learning the language and at one point I could read the news. At that point I could read and understand about 3000 or more characters (and over 10,000 words) and was picking up more every day.
This knowledge gives me an obvious advantage as I can read the characters (kanji) and know what it means most of the time (there are a few exceptions). A lot of times I can even pronounce the character correctly as there is some overlap between the pronunciation (though it has morphed over the many years between the introduction of the characters into the Japanese language). There are also some words that have made it from Japanese to Chinese further aiding in my ability to pick up the language. Coupled with a ton of loan words from English and other western languages, I can understand a lot with little effort (which is great because I do not have much time to spend studying Japanese).
This is not unique to me. Plenty of Chinese speakers can use their knowledge of the characters to learn the language faster than someone who has to start from scratch. I also assume that going from a really solid Japanese background would help you learn Chinese faster as well.
This is just one language pair that has this ability. If you look at the romance languages or any other language family you see this all the time (Chinese and Japanese are not in the same language family). Another great example is English and French. The Norman invasion of England brought a humongous amount of French vocabulary into English that we still have to this day. And this doesn’t just help with learning French as an English speaker or vice versa, all the romantic languages can take advantage of this feature.
So the next time you are studying a language for travel, pleasure, love, or other, take advantage of some shared features between a language you already know and the language you are learning.

