For the Manjaro default theme you could modify it manually for example in /usr/share/grub/themes/manjaro/theme.txt there are localized strings.
But what you want to change is not GRUB theme, it is the GRUB menu itself. Probably that How to change the language settings of grub? - #2 by linux-aarhus or this How to build Manjaro ISO with modified grub parameters? - #4 by RicardoSimoes?
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