It is of course useful for those 700+million, but they are not customers of Google, they users/the product. So long as you won't go elsewhere (in mass) you don't matter.
At any one time, there's got to be tens of millions of people accessing Google from a country which has a primary language unknown to the traveller. Even if this number is insignificant compared to Google's full user base, the cost for Google to service 20-30mn people with a feature is presumably lower than their annual ad revenues across 20-30mn people.