I am attempting to deserialize a YAML config that has mono/multilingual strings in multiple places, such as:

key1: File
# or
key2:
  en_US: File
  de_DE: Datei

Using kotlinx.serialization and charleskorn’s KAML (which adds YAML support for the former), I want to deserialize this into objects such as:

@Serializable
sealed interface LingualString {
    // the actual code uses java.util.Locale; this is simplified
    operator fun get(locale: String): String
}

@Serializable
class MonolingualString(
    private val text: String
) : LingualString {
    override operator fun get(locale: String) = text
}

@Serializable
class MultilingualString(
    private val map: Map<String, String>
) : LingualString {
    override operator fun get(locale: String) = map[locale]!!
}

How do I write a KSerializer<LingualString> that can load such values from the config?