Remove `className={locale}` and add `lang` attribute to `LanguageButton()`

[quiple]

Thanks to the lang attribute in the html tag, the appropriate language-specific font family is automatically applied, so you don't need to specify a language-specific font family (or rather, it breaks consistency, as it turns off Inter in that part)

And I've applied the lang attribute to each language label in the LanguageButton().

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