When looking at translating a game, you need to look at the cost of the translation related to the potential return on investment for adding an additional language.
You also need to take into account the general purchasing habits of consumers in different countries that might predominantly speak a certain language. But, don’t assume someone wants to conduct transactions in French just because they reside in France for example. There is diversity in your target markets and loading up on languages can be very valuable in growing revenues.
Most people are more comfortable buying games, or purchasing In-App Purchases if their primary language is supported. Having conducted a transaction in French on Steam recently, I could get through it, and make the purchase because the flow was the same as when I purchased in English before, but there were more opportunities to abandon the purchase because it took longer to comprehend the steps.
So I pulled the Steam Hardware survey info and Added a column for what percentage of the Steam player audience you get by adding each new language up to the 12th most popular language.