One's mastery over a language cannot be tested just by his capacity to speak, read or write in that language..
He is good at a language only if he can form thoughts too and arrange mental and visual ideas in the language in which he is trying to express himself..
If he is using more than one language, he should be able to keep away the influence of the languages other than which he is applying for the present
we see funny pieces of literary and verbal acrobatics submerged in absurdity because the user just thinks and sees only in his own favourite langauge and is trying to give expression to such thoughts and visions by some sort of translation to another language..
Actually many Indians are multilingual..
For example a palakkad iyer can easily think separately, as the occasion demands, in Malayalam, Tamil, English and a little Sanskrit too..
And the Mumbai Iyer of Kerala Origin adds Hindi and often Marathi too to this..
And a Bangalore Kerala Iyer will adopt Kannada too like this.
Of course people with messy thought process and having smatterings of vocabulary of various languages create hybrids.. funny as they are..
But exclusive and water tight linguistic processing by the mind in thoughts and actions is actually very easy..
Translation can be hazardous..Thoughts lose their life and grammar too in the process..
However, this will apply to people who want to think and transact their business in an orderly manner..
Nothing applies to great jnaanis and absolute morons.