Allowing different languages on the site would cause several problems.
SE sites are fundamentally not set up to handle such divisions into different subsites. Tags are a poor substitute, and don't work all that well to filter out languages. Tag would be a very bad solution especially to new users who would not know how to use them to filter out specific languages, misusing the tag feature this way would be very confusing.
The experience for new users on the site would be that they see questions in different languages, but no obvious way to only show english questions. This would leave a very bad impression to new users.
But the bigger issue is that it would create a dangerous bottleneck in moderation. As only a subset of the users and moderators speak spanish, far fewer users would be capable of moderating content in spanish. You really need most of the moderators to be able to act on all content on the site due to differences in the moderators timezones and vacations. Together with the inability for moderators to filter the flag queue for different languages this would make moderation in general more cumbersome, and there is no guarantee that the foreign language content could be moderated effectively.