The library author did answered the question - translations defined as part of Vue SFC are compiled (by the loaders for appropriate build tool) as an export from SFC module. That means they are part of the component module and only way to lazy load is to lazy load the component itself
So no, there is no way to lazy load single language defined in i18 custom blocks. If you need lazy loading, don't use custom blocks and rather define your translations in global JSON files (split by language). Other alternative is to completely rewrite the loader itself to collect and merge custom blocks into such global module but that is a lot of work and I would not recommend that...