One-way language?

The brain plays a vital role in language learning. You learn a new language by memorizing words. When you speak it, you use your memory to retrieve the words that you have learned. Thus, Memory is essential to all learning, because it allows for storage and retrieval of information. So, acquisition of new skills involves brain function of input process, data store and data retrieval.

Parents outside India struggle to make their kids speak Kannada. In some household parents speak to kids in Kannada but kids respond only in English. Kids can respond quickly to English translation of a Kannada word, but they struggle or may not know the other way. Since parents speak Kannada, the kids brain stores the information. Kids mostly find the easy way. If the data retrieval is not exercised, the data if still exists in the brain cannot be easily retrieved. As they get older their ability to reproduce diminishes. Even if they can retrieve, they feel shy of saying it as they never practiced the sound reproduction. The sound reproduction is a muscle memory, that needs to be practiced.

Can English language (first language) be used to learn Kannada (the second language)? is that effective?

Learning depends heavily on memory, the knowledge already stored in your memory provides the framework to which you link new knowledge, by association. The more extensive your framework of existing knowledge (first language), the more easily you can link new knowledge (Kannada) to it. Learning is a relatively permanent change in behavior that marks an increase in knowledge, skills, or understanding thanks to recorded memories. Human memory is fundamentally associative. It is easier to remember new information if it is associated with previously acquired knowledge that is already firmly anchored in memory. The more meaningful the association, the more effectively it will help remember the second language.