Samsung commenced Marshmallow rollout for the Galaxy S5 in South Korea (SM-G900L) late last month, and the update is now hitting the global variant of the phone (SM-G900F). The SM-G900F variant is sold in Europe, Latin America, and parts of Asia, with the update rolling out to users in Sri Lanka.
Samsung is known to roll out updates in countries with a relatively minor userbase, following which they're made available widely. It looks like that's the case with the Galaxy S5 as well. There's no timeframe as to when we'll see the update becoming available in other regions, but we'll let you know once we hear more.