The standard is, for better or worse, gettext; it's good enough that any attempt to replace it runs into the problem that people can't agree on how much better

The standard is, for better or worse, gettext; it's good enough that any attempt to replace it runs into the problem that people can't agree on how much better an alternative needs to be to be worth migrating to; so you get a constant churn that so far hasn't seen any clear winner.