P.S. When I changed the sentence to "It's very difficult to get into a language, when you're thinking in two languages at once", he said it's okay.
You know Olya, most people would not notice from your written English that you are not a native speaker. I wouldn't.

I just googled "penetrating language" and it turns out that you CAN say it, but it is not commonly used. Not a very good thing to use in everyday communcation....
http://www.enotes.com/shakespearean-...ating-language
http://www.eric.ed.gov/ERICWebPortal...accno=ED296611