Different languages often require you to use different fonts, there are very few fonts that contain characters for all languages. Ranges using specific languages are tagged with the 'lang' attribute so the browser can use the appropriate font. If you aren't allowed to include any font face rules you lose the ability to handle multiple languages (unless all the languages you care about use codepoints below 255).
Italic and Bold are also separate fonts (albeit in the same family).
Internationalization and localization don't require more fonts, just different fonts. When a user is viewing a site in English they'd see a different font to a user viewing a page in Japanese. That's fine. Just serve one a stylesheet with the English font first, and the other a site with the Japanese font first. They would still work fine.
Bold and italic were different fonts in the past, but variable fonts have solved loading separate fonts for different weights and italics. This change would require developers swap to use those in many cases, and to create variable versions of brand fonts if necessary. I don't see that as a problem. The benefit makes it worthwhile.
"Viewing a site in English" misunderstands the problem. Mixed-language content is a reality. Not all websites are a single language, because they can host user-generated content or snippets of content in other languages (commentary on translated material, live translations, instructional material, etc.)
Italic and Bold are also separate fonts (albeit in the same family).