Most bilingual projects are monolingual projects with a translation phase bolted on at the end. That ordering is the reason so many of them look broken in the second language.
Layouts fail in the language you did not test
Spanish runs longer than English — often fifteen to thirty per cent for the same sentence. A navigation bar, a button, a card heading and a table column that all fit comfortably in English will wrap, clip or overflow in Spanish. If the first time anyone sees that is the week before launch, the fix is not a translation fix. It is a layout fix, on a design that has already been approved.
Building both from the start turns that from a crisis into a constraint. You design the card knowing the heading has to survive a longer word, and you find out on the day you build it rather than the day before you ship.
It changes what you write, not just what you translate
English lets you lean on idiom and compression. When every sentence has to work twice, you write more plainly the first time — and plainer English is better English for the audience that reads it, too. The discipline improves the original.
Make the machine enforce it
Good intentions do not survive a deadline. On this site a key added to the English copy and forgotten in Spanish is a typecheck failure, not a blank space a Spanish-speaking visitor discovers. A test also fails if a Spanish value is still the English sentence, which is exactly what an interrupted translation pass leaves behind and exactly what nobody notices in review, because the page renders fine.
Machine translation is a starting point, never the finished thing. A qualified speaker still reads it before launch. On a site that sells multilingual delivery, machine-shaped Spanish beside that claim disproves it.