At least in 0.212.1, .toLocaleString('en-US') works just fine.
What used to work and doesnt work anymore, is the mentioned format .toLocaleString({weekday: ‘long’}), or toLocaleString(DateTime.TIME_24_WITH_SECONDS). What I think happened, is that this previously valid syntax is not valid anymore, and now you have to pass the locale as the first parameter, even if it is empty.
So: .toLocaleString('en-US', {weekday: ‘long’}) and even .toLocaleString('', {weekday: ‘long’}) are both valid. For some reason, toLocaleString(DateTime.DATE), without the locale parameter also seem to be valid…
I do believe this is unintended or at least not well documented behavior, as it is a breaking change and the (now) invalid syntax is mentioned in documentation:
I was just ruling out a general failing, I did see your number and hit the same issue but I also can’t work out what format that number is which would be a big help in working out where it is going wrong.
Ah ok, That explains it. I have only ever seen .toLocaleString used with dates not numbers. I have just given that a test and it appears to work as well.