I always design i18n with a preferred language on a customer profile and I use Accept-Language to pre-select it on registration. Browser settings must be respec

I always design i18n with a preferred language on a customer profile and I use Accept-Language to pre-select it on registration. Browser settings must be respected, of course, but there are also non-browser communications that can use this preference. It is very small amount of work if done at the start of the project rather than as feature upgrade.