One problem is that English is the language majority of technology is created in, and it has developed agreed-on terms for a lot of things. Everyone calls their tabs "tabs" in English, but when it comes to translating, every other app uses a different word. Even if names are consistent within the app, they are inconsistent between each other. This leads to a lot of problems if say, an app reports an error that's related to some OS thing, and calls it with a different translated name than the OS translation does.

EDIT: And I'm not making this up. I worked on a localized Ubuntu for a very short while before switching back to English because messages I got in my command line were painfully inconsistent with each other and with agreed-upon proper translations in my language.