Thailand, famously, was never colonized by European powers. Everywhere else, some colonial administrator standardized a system of romanization.
Sri Lanka was a colony and Sinhala does not have a standard as far as I know. If there is one no one pays any attention to it.
Oh there are plenty of standards, including an official one. The problem is nobody uses them. Thai writing is weird, and between the tones and the character classes and silent letters might as well just make some shit up. My birth certificate, drivers license, and work permit all had different spellings of my name on them.
IIRC, the road signs for “Henri Dunant Road” were spelled differently on either end, which was ironic, because at least that did have a canonical Latin form.
Japan was not colonized, although it was briefly occupied.