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.

The Bournemouth striker, who has been included in a 27-man final squad, has yet to receive official clearance from Fifa to play for the country of his parents.
The 23-year-old has represented England at youth level since 2005, having captained the Under-12s, won the Victory Shield with the U16s, and turned out for the U17s, U19s and U21s.
Afobe is expected to join up with his prospective teammates next week in Kinshasa, but Fifa is yet to approve his application to switch nationality under world football's governing body regulations.
According to the rules, players who have represented one country at junior level can play for another at senior level if they meet the qualifying stipulations.
He holds dual nationality and has not played for England's senior side, making him eligible for the Leopards - once an application has been made to Fifa and approved.
Afobe, who moved to Bournemouth from Wolves for £10m in January, has scored four goals in 10 appearances for the Cherries so far this season.
The former Arsenal striker scored 23 goals in two seasons with Wolves.
Meanwhile, the DR Congo squad will officially commence their preparations at Béatrice de la Gombe hotel in the capital on 20 March.
They are currently third in Group B after a home win over Madagascar and a shock 2-0 defeat at Central African Republic in September.
CHAN winner Florent Ibenge, who also guided DR Congo to third place at last year's Africa Cup of Nations in Equatorial Guinea, is desperate for positive results against group leaders Angola.
Only group winners are guaranteed automatic qualification to the 2017 tournament in Gabon.
The Leopards have also progressed to the final round of qualifying for the 2018 World Cup.
A film by valleys directors shot in a language they did not understand with orphaned children has won an award.
[NEXT_CONCEPT]
Ex-England youth international Benik Afobe will not play for DR Congo in next weekend's 2017 Africa Cup of Nations clash with Angola.