mixing custom language

What I want:
I want to mix in the same file a custom language with java (similar to plugin vuejs), having a limiter for each language.

ex:
[custom]
// custom code
[Java]
// java code

My progress:
Searching the source code of the vue plugin, I found LanguageSubstitutor (I don't know if I'm on the right track).
I didn't make progress with LanguageSubsitutor because I can't identify the limiters via bnf

Doubts:
Is there any documentation that helps in mixing languages?
Can someone give me a north?