The Bench was written in English, but the script was changed to Polish and actors with no experience enlisted from the town of Legnica.
Green Valley Films' Kevin Phillips had said it would be a "miracle" if it triumphed at the London International Filmmaker Festival.
But it won best original screenplay of a foreign language film on Saturday.
The film was written by a group of friends from Blaenau Gwent - and was their first attempt at writing a movie.
But after encountering logistical problems with closing streets in the valleys to film, they decided to shoot in the town of Legnica - where Mr Phillips had filmed a documentary.
They changed the script to Polish and enlisted children from a local special school and orphanage, with pupils from two Blaenau Gwent schools flying out to be extras.
Filmed for just Â£4,000, with all involved giving their time for free, it had been listed in the best original screenplay of a foreign language film and talented new filmmakers categories.
Looking at the budgets of the other films, from countries such as Japan, the US and Germany, Mr Phillips said it would be "a miracle" if it won.
He said they had just gone to the awards "for the experience", adding: "It was our first movie, shot totally independently, nobody had ever acted.
"It brought so many people together with everyone working for free. It was a terrific project."
With about 30 children from Pen y Cwm special school and Deighton Primary, Blaenau Gwent, flown out to be extras, he said it was a good "concept film", an example of how communities could establish bonds through film.
Judges said international exposure from winning could help promote it in the UK, Poland and other countries.
It is due to have a premiere in Legnica in April and has also been entered in Hollywood's Polish Film Festival.
A film by valleys directors shot in a language they did not understand with orphaned children has won an award.