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I think the origin of separate languages is distance (thinking of several arguments written in Guns, Germs, and Steel). With instant worldwide communication the issue seems more inertia than distance. Same reason we Americans haven't really adopted the metric system. Seems to me a far future SF novel has to pick one of two paths. If galactic communication is instant then a common language is reasonable. If not, or some group is intentionally isolated or nationalist (the trope of the Japanese colonists and the American colonists), then there might be a common trading language but local dialects would have reason to continue.
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