G10 currencies markets have active FRA markets and apart from NOK, SEK and NZD one can also speculate on interest rates through exchange-traded futures. What's the best liquid way to speculate on interest rates on 3mo to 3 or 5 year horizon in currencies like BRL (for someone without access to BMF Bovespa), MXN, CLP, COP, ZAR, PHP, MYR, IDR, PLN, HUF, TRY, CZK, RUB? Do market speculants usually trade FX/NDF swaps or prefer other OTC products for such exposure in any of the currencies?
Interest rate swaps for 2-10Y and forward rate agreements for the short end (or the non-deliverable variant, where applicable) are the best way. Some currencies are available via RFQ on aggregators like TradeWeb (e.g. ZAR, PLN, HUF, CZK) but others are very much voice markets. You’ll need a prime broker, and give-up agreements with executing brokers to get the best liquidity.
You could trade the ICE/CME futures in FX, but like you said the liquidity is not very good. Outside of having a relationship with an I-Bank (read need ISDA or Prime Brokerage), getting access to the OTC market is practically impossible. You could try and find ETFs which hold the underlying you're interested in. You could also try and find a broker who will write you these swaps, but if you are a retail trader, this will be tough.