[savio marel oliver (OP)]

In what language does a person think, if he or she knows more than 1 language?
Is it their mother tongue or the language used just before thinking.

Example:
In this hypothetical case , your mother tongue is French and your friend ask you in English and your reply will be n english. What language will you think to reply, in this case, English or French ?
 
Suppose your friend ask you to come to the bar, as he giving you a treat and then you think "whether you will be free or not". That is for those who need to think  []. 

[Atomic-S]

well, I really don't know for sure; my suspicion is that it depends on the person, and that in general the brain will tend to jump around into whatever language(s) are most amenable to the subject matter and/or accessible at the moment. And this probably has a lot to do with the person's familiarity with the languages.

[Titanscape]

What about thinking in pictures. Then we compose words in a language. One lecturer at Sydney Uni, Alan Snyder teaches alternate uses of different parts of the mind for the same purpose, or various. He won a prize from the PM for an invention of a new cable network based on the insides of the eye of an insect.
http://www.centreforthemind.com/whoweare/index.cfm

Some neo pagans think that the language teaches the basis for your beliefs and values... like German and the society of Woden. Others like myself suggest the Gospel is ingrained so much so that atheists are much different to ancient ones.

There are also different personality types apart from introverts and extraverts, like melancholics... they are all normal.

Perhaps Freudian slips may suggest something of how some think.