Is sub-vocalization when reading and writing merely a symptom of lack of fluency? In reading &amp; writing, I for one, find myself subvocalizing the statement to myself. This behaviour is language-independent. However, the effort and subvocalization involved is greater in any language that I do not use regularly.

Is sub-vocalization when reading and writing merely a symptom of lack of fluency in a language?
My understanding was that at least some sub-vocalisation is a normal part of reading and writing. The wikipedia article on subvocalisation cites several sources supporting that claim. The article also claims that there is no evidence to suggest that speed reading training that involves suppression of sub-vocalisation is effective. There is also evidence that suppressing subvocalisation can in some instances result in decrements in performance (see Baddeley et al, 1981).

There is some evidence to suggest that degree of subvocalisation is related to complexity of the reading passage. Baddeley et al 1981 summarise the use of


  the  electromyographic monitoring of  the muscles involved in
  vocalisation  in order to study the role  of  articulation in  reading
  (e.g. Sokolov,  1966).  Using  this technique,  Hardyk  and 
  Petrinovitch  (1970) concluded  that subjects were  able  to read  and
  comprehend  simple prose  without  the  occurrence  of 
  electromyographic activity,  but that such  activity  did  occur in
  the case of  more complex passages.


One explanation would be that degree, prominence or frequency of subvocalisation increases as the reading task gets more difficult, where experienced difficulty would vary both based on the individual's ability with the language and with the objective difficulty of the reading passage.

References


Baddeley, A.,  Marge Eldridge &amp; Vivien Lewis (1981): The role of
subvocalisation in reading, The Quarterly Journal of Experimental Psychology Section A: Human Experimental Psychology, 33:4, 439-454
HARDYK, C.  D.  and PETRINOVITCH,  L.  R.  (1970).  Subvocal speech  and  comprehension  level as a function of the difficulty level of reading material.  Journal of  Verbal Learning  and Verbal Behavior, 9, 647-52. 
SOKOLOV,  A. (1966).  La parole intbrieure dans la pensbe concrete. In Recherches Psychologiques en U.R.S.S.,  les Editions du Progrks, U.R.S.S.