Reddit is an American company and is not culturally equipped to manage a community-based site that isn't Amerocentric. There have already been countless conflicts between the administration and the userbase. Those conflicts would increase to the point of being an almost constant distraction if the same small and somewhat tone-deaf team is forced to grapple with meaningful cultural diversity.

Reddit is also primarily an English-language site, and its design, which is fundamentally reliant on aggregating content from different communities, would simply not work if a large number of languages were in regular use; it would make the front page, /r/all, and multireddits incredibly frustrating for everyone but extreme polyglots.

Linguistic diversity is an inevitable product of emphasizing cultural diversity. Just look at the example of /r/sweden - the vast majority of Swedes speak perfect English, yet /r/sweden prefers to converse in Swedish (and why wouldn't they) and loves it when they upvote Swedish posts to the top. Imagine if there were even 20 other countries with that kind of presence. The front page would be unusable, and the fights between mods and users (and ultimately admins) over posting rights in different languages in other subreddits would be endless.

Ensuring that English remains sufficiently predominant on the site, and as a corollary that the site isn't shit, means accepting a large degree of Amerocentrism, since roughly two-thirds of native English speakers are American and the vast majority of English-language media is American.