On the subject of the success of the Witnesses' "foreign field", I have not found much discussion of it in the academic literature, apart from a short extract of a paper delivered by Richard Singelenberg at a conference:
In a session on "Recent Developments in Western Movements," Richard Singelenberg of the University of Utrecht (Netherlands) presented a paper on The Changing Face of Jehovah's Witnesses. Singelenberg - who lamented the fact that so very few social scientists are seriously studying the Watchtower movement - took issue with the recent article by Rodney Stark and Laurence Iannaccone in the May 1997 issue of Journal of Contemporary Religion ("Why Jehovah's Witnesses Grow So Rapidly: A Theoretical Application"). He especially questioned their optimistic growth projections for the movement, giving persuasive statistical evidence of a steady decline in Watchtower conversions worldwide (except for Eastern Europe). Singelenberg commented on the consequences of "free-riding" - the trend toward nominal membership (i.e., inactivity or inconsistency among Witnesses in fulfilling their mandate to proselytize). He challenged stereotypes that the Watchtower is winning most of its converts from among people in lower socioeconomic strata and noted that, although the Witnesses put great emphasis on proselytizing among immigrants and refugees, he is unconvinced of their success.
From my very limited observations I would say the success is mixed, and that certainly the results are not equal to the tremendous efforts that some Witnesses are putting into the "foreign field". They seem to be doing better among Chinese people and worse among Indians/Pakistanis. I would suggest that this is because Chinese people often come to the UK alone to study, and Witnesses may therefore catch them at a time when they are a bit isolated and looking for friendship. Indians/Pakistanis on the other hand maintain very strong familial networks here in the UK and it is much harder for the Witnesses to break into that. The exception I know of proves this rule: a young Indian man was baptised while he was living in a rural part of Scotland, alienated from his father whom he had fallen out with. When the man went down to England where more of his relatives lived, I think he stopped attending meetings.
There is no doubt the Witnesses are having some success with their foreign field work, the question is how much, and whether it justifies the effort being expended. I remember at the convention last year it was claimed the number attending foreign language congregations has gone up from something like 1000 to 6000 over the past ten years. A good number of that increase is doubtless British Witnesses who are trying to learn another language. On the other hand, the fact that the number of foreign language speaking Witnesses is increasing also indicates that the static UK publisher figures are somewhat masking a more precipitous decline in English speaking Witnesses than they would at first suggest.
I am convinced that the real value of all the effort being put into the foreign language field from the organisational point of view is that it increases the commitment of British Witnesses who take part in that activity rather than the actual increase in foreign language speaking Witnesses.