There are a lot of things that change - lipsync, the timing of some of the animations, some textures are swapped so the text on the texture is in the other language instead of English (stuff like the weapons BoscoTech logo), in-game UI screens (we would have had to resdesign the UI so there would be space for a language selector, and a programmer would have had to make it work...) Plus it would have meant that instead of creating a separate install for each language, which was fairly easy for us to do, we would have had to create one mega-install that included all of them, which would have been a lot more complicated and would have required quite a bit more testing.
You're right that being able to switch between languages after installation would have been a better user experience, but we had to evaluate what we had the resources to do against how important it was, and based on the amount of work and time it would have taken to implement a relatively minor convenience, a decision was made to do it this way. Sorry it means having to uninstall and reinstall.
(That's probably a lot more information than you wanted, I just don't want anyone to think we're idiots who don't realize that uninstalling and reinstalling in another language isn't the most convenient way to switch between languages. We're not idiots and we do realize that.

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