Can I use neem leaf for substitute of hops? Especially it's difficult to manage hops in our region. I found a lot of health benefit in neem leaf.
You can substitute any bitter herb for hops if you wish. Also given the fact it has anti-fungal and anti-bacterial properties, it will likely protect your brew from biological contamination, as hop alpha acids do.
As it is anti-fungal it may interfere with your yeast. Also, I do not know what off flavours yeast may produce when fermenting in the presence of Neem tea compounds. You may be fine, but experience with mead made from eucalyptus honey makes me consider this as a possible adverse affect.
If you do try this please report back.
Before hops a wide range of herbs are recorded as being used to bitter/flavour ale/gruit there are a few modern breweries doing this: https://en.wikipedia.org/wiki/Gruit