If they had developed industries earlier, nowadays the country would not be so poor.
I think it's always best to go by your French rules:
si+pluperfect, conditional perfect
The correct sentence would be: "If they had developed industries earlier, nowadays the country would not be so poor."
It's almost an exact translation from French: If they had developed= S'ils avaient développé.
In informal conversations, it is also possible to use "would have" where a plus-que-parfait is technically grammatically correct: "If they would have developed...." But this should only be used for informal conversations, never for formal writing.