> but then you're the only one that knows your language

This only applies if the language implementation is complex, the language syntax is unlike anything in existence, and the language creator does not pick good names for functions and variables nor write any documentation, manuals, or papers to explain things nor he/she is available for any questions.

I don't think that's true. What you describe is a language that's hard to learn. But plenty of people already have a moderatly hard time when switching languages, or when using languages inside languages. One example would be SQL. SQL is a great DSL for relational databases, but many people never really embrace it or learn it "properly".