@Andrew *******
Well… It’s not forbidden to translate the first page. But it’s not mandatory. If you look well at those translated IDP, and you seem to have seen hundreds or so, then of course you have noticed that the non-native language is in Italic print. As mandated by the treaty.
The treaty specifically states that the first page must be in (one of) the native language of the issuing country. Additional translations (e.g. Thailand provides an *ADDED* English translation) must be in Italic print.
Go find the treaty text if you think you know better.
PS You may have seen an American or a British IDP. These countries are (sort of) native English.