Q:

Do people think in a language?

I was discussing some things with a psychology major, and he insisted that people always use a language to think. This is quite opposed to my own experience. 
I agree that I am capable of formulating my own thoughts in a kind of internal monologue, which is certainly in a language. But this is just one kind of my thought process. Sometimes my thoughts seem to be less language-bound. And what I think is the most important, it happens to me sometimes that I am speaking and suddenly notice that the word I am going to say is in a different language (almost always it happens just before I say it, but after the sentence has been formed and said up to the word). I don't mean the cases where I have learned a concept in another language and I am grasping for the correct word in the language I am currently speaking, I mean perfectly everyday words, like saying "I saw the book " and realizing that the word which I am going to say next is "gestern" instead of "yesterday". But at the time I realize this, I have already spoken the preceding part of the sentence. In very rare cases, I only notice it after I have said it, and hear my own wrong sentence. 
I interpret such occasions as follows: I must have thought of the time reference without using a word in a language, after that constructed a sentence without consciously choosing words (else I would have noticed that "gestern" is wrong), and only made use of my vocabulary after that, practically at the point of commanding the mouth to form the words. 
But he claimed that this isn't true, and that humans always use a language for thinking, not just for communicating. He couldn't point me to sources, or even tell me about an author researching such problems. He just claimed that he knows it for a fact, and must have learned it in a lecture. Do you know of research in that area? And what is its conclusion?

Skivvz's comment about off-topic makes me think that maybe I didn't state my question clear enough. 
The claim I am disputing is: People always use a language in their internal thought processes. I provided an example which I interpret as anecdotal evidence against the claim. I also explained my interpretation. I am not asking how good my interpretation of this example is (this is probably the content suited to a psychology forum). But if you know of research which proves or disproves the claim, I'd like to hear about it. 

A:

No. Human thought precedes language.
The anecdotal evidence for this should suffice, but you cannot be trusted (as you already have language skills). 

Short Answer: Prelingual infants think.
In 2004 researchers Hespos and Spelke explored Korean language concepts with a group of five-month-old (human) infants from English-speaking homes...

The example they used to explore this
  question was differences between how
  different languages describe space.
  For example, the distinction between a
  tight fit versus a loose fit is marked
  in Korean but not in English. A cap on
  a pen would be a tight fit
  relationship, while a pen on a table
  would be a loose fit relationship.
  English does not mark this distinction
  in the same way, instead emphasizing
  the “containment” versus “support”
  relationship, for example: the coffee
  is in the mug or the mug is on the
  table. - source

...the infants showed an understanding of events that represented a change in "fit"... 

Because this capacity is observed well
  before the acquisition of a natural
  language in infants whose ambient
  language does not mark the
  distinction, this capacity does not
  depend on language experience.
  Instead, the capacity seems to be
  linked to mechanisms for representing
  objects and their motions that are
  shared by other animals and therefore
  evolved before the human language
  faculty. - source

In other words...

Learning a particular language may
  lead us to favor some of these
  concepts over others, but the concepts
  already existed before we put them
  into words. - source

More: Why would you think language is required ?

On one hand we can claim that we can
  even think in pictures or on the other
  hand one has to think to learn a
  language. In terms of neurosciences it
  has been proved that thinking without
  language is possible. However the
  philosophical references often deny
  that one can think without language.
  - source

These philosophical discussions have been going on for sometime...

When people have begun to reflect on
  language, its relation to thinking
  becomes a central concern. Several
  cultures have independently viewed the
  main function of language as the
  expression of thought. Ancient Indian
  grammarians speak of the soul
  apprehending things with the intellect
  and inspiring the mind with a desire
  to speak, and in the Greek
  intellectual tradition Aristotle
  declared, “Speech is the
  representation of the experiences of
  the mind” (On Interpretation). Such an
  attitude passed into Latin theory and
  thence into medieval doctrine.
  Medieval grammarians envisaged three
  stages in the speaking process: things
  in the world exhibit properties; these
  properties are understood by the minds
  of humans; and, in the manner in which
  they have been understood, so they are
  communicated to others by the
  resources of language. Rationalist
  writers on language in the 17th
  century gave essentially a similar
  account: speaking is expressing
  thoughts by signs invented for the
  purpose, and words of different
  classes (the different parts of
  speech) came into being to correspond
  to the different aspects of thinking.
  - source

Wilhelm von Humboldt: Credited as an originator of the linguistic relativity hypothesis (aka: the Sapir–Whorf hypothesis).
Benjamin Lee Whorf: Widely known for his ideas about linguistic relativity, the hypothesis that language influences thought.
Noam Chomsky: Well known in the academic and scientific community as one of the fathers of modern linguistics.

On the one hand, most people, after
  hearing evidence that language is an
  innate faculty of humans, would not be
  surprised to learn that it comes from
  the same source that every other
  complex innate aspect of the human
  brain and body comes from — namely,
  natural selection. But two very
  prominent people deny this conclusion,
  and they aren't just any old prominent
  people, but Stephen Jay Gould,
  probably the most famous person who
  has written on evolution, and Noam
  Chomsky, the most famous person who
  has written on language. They've
  suggested that language appeared as a
  by- product of the laws of growth and
  form of the human brain, or perhaps as
  an accidental by-product of selection
  for something else, and they deny that
  language is an adaptation. I disagree
  with both of them. - Pinker, Language Is a Human Instinct

Steven Pinker: Argues that language is an "instinct" or biological adaptation shaped by natural selection.

...These scholars, ranging from
  Aristotle to Freud, took these
  specific instances to be exceptional,
  marginal eruptions of meaning, curious
  and suggestive. But none of them
  focused on the general mental capacity
  of blending or, as far as we can tell,
  even recognize that there is such a
  mental capacity. Attentive to the
  specific attraction - the painting
  ,the poem, the dream , the scientific
  insight - they did not look for what
  all these bits and pieces have in
  common. The spectacular trees masked
  the forest.
  - Turner-Fauconnier, The
  Way We Think

More...

Does language shape what we think ?
How does our Language shape the way we think ? 

Background: Why is language important ?
Before Homo sapiens came on the scene 150 kya innovation and change amongst the genus of the family Hominidae was pretty dull. Homo habilis showed up 2.4 mya and stayed around for a million years. They had one, and only one, great idea: Stone tools. Next up was Homo erectus (1.5 – 0.2 mya). Erectus was a slow starter but about 400,000 years ago they hit pay-dirt: Controlled use of Fire. Great. For 2.2 million years of effort we have some sharp rocks and a barbeque.
Then things get really interesting.....

Any innovation must take place within
  a species, since there is no place
  else it can do so. Natural selection
  is, moreover, not a creative force. It
  merely works on variations that come
  into existence spontaneously—it cannot
  call innovations into existence just
  because they might be advantageous.
  Any new structure or aptitude has to
  be in place before it can be exploited
  by its possessors, and it may take
  some time for those possessors to
  discover all the uses of such
  novelties. Such seems to have been the
  case for Homo sapiens in that the
  earliest well-documented members of
  our species appear to have behaved in
  broadly the same manner as
  Neanderthals for many tens of
  thousands of years. It is highly
  unlikely that another species
  anatomically indistinguishable from
  Homo sapiens but behaviorally similar
  to Neanderthals was supplanted
  worldwide in an extremely short span
  of time. Therefore, it seems
  appropriate to conclude that a latent
  capacity for symbolic reasoning was
  present when anatomically modern Homo
  sapiens emerged and that our forebears
  discovered their radically new
  behavioral abilities somewhat later in
  time.
A cultural “release mechanism” of some
  sort was necessarily involved in this
  discovery, and the favoured candidate
  for this role is language, the
  existence of which cannot be inferred
  with any degree of confidence from the
  records left behind by any other
  species but our own. Language is the
  ultimate symbolic activity, involving
  the creation and manipulation of
  mental symbols and permitting the
  posing of questions such as “What if?”
  Not all components of human thought
  are symbolic (the human brain has a
  very long accretionary, evolutionary
  history that still governs the way
  thoughts and feelings are processed),
  but it is certainly the addition of
  symbolic manipulations to intuitive
  processes that makes possible what is
  recognized as the human mind.
The origins of this mind are obscure
  indeed, especially as scientists are
  still ignorant of how a mass of
  electrochemical signals in the brain
  gives rise to what we experience as
  consciousness. But the invention of
  language would plausibly have released
  the earliest of the cultural and
  technological innovations that
  symbolic thought makes possible—in the
  process unleashing a cascade of
  discoveries that is still ongoing. One
  of the most striking features of the
  archaeological record that accompanies
  the arrival of behaviorally modern
  Homo sapiens is a distinct alteration
  in the tempo of innovation and change.
  Significant cultural and technological
  novelties had previously been rare,
  with long periods of apparent
  stability intervening between
  relatively sudden episodes of
  innovation. But once behaviorally
  modern Homo sapiens arrived on the
  scene, different local technological
  traditions—and, by extension, other
  forms of cultural diversity—began to
  proliferate regularly, setting a pace
  that is still gathering today.
  - source

The Bottom Line 
When you look into the eyes of the one you love and think of what they mean to you...
What language do you think in ? 
Love is a smoke made with the fume of sighs, Being purged, a fire sparkling in lovers’ eyes, Being vexed, a sea nourished with lovers’ tears. What is it else? A madness most discreet, A choking gall and a preserving sweet. - Bill
What language was he thinking in ?

A:

Your question reminded me TED talk:
http://www.ted.com/talks/temple_grandin_the_world_needs_all_kinds_of_minds.html
in which Temple Grandin explains how her autistic mind works. She says that unlike majority of people she thinks in pictures, not in language.

