True, but English is the mother tongue of a small minority of the human population, and as developing countries get wired up to the Internet, the percentage of English readers will decline. To penetrate these foreign markets one must speak the customer's language. Some countries, by law, require manuals to be in their official language. Hence, translating manuals is often necessary in order to sell a product in a foreign market. Some languages represent bigger slices of the market, and this should set the priority of which languages to translate into.
Many international organizations spend a fortune to have stuff professionally translated.
It has been my experience that these professional translators do a poor job. On several occasions I had to completely rewrite translations because of grammatical and contextual errors in the translation. Explaining the context in which things are used, including giving the translator a product demonstration, helps to improve the accuracy of the translation. Unfortunately, this rarely happens because translation companies discourage communication between the client and the translator. They fear that communication back and forth between client and translator will cause the job to run over budget.