there is a "not" before "to", that means, it is a double negative?
No, a double negative would be something like this:
We ain't going to no party.
Which is really saying "we are not going to no party" which, according to the sentence, means they are going to a party. See this Youtube video for additional examples of double negatives.
The plan is complicated enough so we must do what?
The plan is complicated enough so that we must involve the "interests" and "local institutions" that were previously mentioned.
The long-winded sentence is basically saying that if we do not involve the people and institutions that will be affected by the plan, it will be a bad thing since the plan will affect so many others.
Best Answer
"few" is used with a countable noun: "There are too few apples in the box". This is equivalent to "not enough apples".
"Water" is not countable, so you cannot say "too few water".
You could use "little": There is too little water in the glass". This is equivalent to "not enough"