Laddering is when you learn a new language by using resources in the previous language you have studied. For example, when you use your second language (L2) to learn your third language (L3) instead of using your native language to learn any new language. Khatz talked about laddering on the old AJATT site.
That's how I learned Japanese. I exclusively used resources in my L2 (English). If you have learned English as a foreign language, and you want to learn Japanese, you should definitely use English resources to do that. English has many good resources for learning languages.
I wouldn't recommend using your L2 just because it's your L2. Use the language that has the best resources. Usually, but not necessarily, it's the closest language to your target language (TL). For example, you can use Korean resources to learn Japanese or Russian resources to learn Ukrainian, assuming you know those languages of course. The idea is that the closer a language is to the target language, the more accurate it can be at describing the target language.
Another point is that if you keep using L2 resources for the sake of laddering, it will slow you down. The best way to learn any language is by using resources made in the language itself. Go monolingual as soon as possible by switching to resources and dictionaries made for native speakers of the target language.
Until you go monolingual, when looking up a word, reading bilingual definitions in several languages will give you a much better understanding than reading a dictionary in just one language because in each language there will be a slightly different translation of the word. When you fuse all the angles together, you get a more comprehensive view of the word.
Laddering is often presented as a tool for maintaining your L2, but it does a very poor job of doing so. The only time you are going to be contacting your L2 is when you look up a word, and when you read a grammar guide. When you're learning a language through input, all your time is spent immersing, you're not spending that much time with the base language. After you go monolingual, you completely stop getting exposure to the L2.
To summarize, when applied literally, laddering is rarely helpful unless your L2 is closer to your L3 than your native language. It is more helpful to use all your languages to find the best resources.