Wrapper for strings without Android-dependency

LocalizedString.kt

package ...

private val identityMap: (String) -> String = { it }

sealed class LocalizedString

data class ResourceString(
    val id: Int,
    val quantity: Int? = null,
    val args: List<Any> = emptyList(),
    val map: (String) -> String = identityMap
) : LocalizedString()

data class Wrapped(val stringValue: String) : LocalizedString()

data class CompoundString(val strings: List<LocalizedString>) : LocalizedString()

val String.wrap get() = Wrapped(this)

operator fun LocalizedString.plus(another: String): CompoundString = this + another.wrap

operator fun CompoundString.plus(another: String): CompoundString = this + another.wrap

operator fun LocalizedString.plus(another: LocalizedString): CompoundString =
    when (this) {
        is CompoundString -> when (another) {
            is CompoundString -> CompoundString(this.strings + another.strings)
            else -> CompoundString(this.strings + another)
        }
        else -> when (another) {
            is CompoundString -> CompoundString(listOf(this) + another.strings)
            else -> CompoundString(listOf(this, another))
        }
    }

operator fun LocalizedString.plus(another: CompoundString): CompoundString =
    CompoundString(listOf(this) + another.strings)

operator fun CompoundString.plus(another: LocalizedString): CompoundString =
    CompoundString(this.strings + another)

operator fun CompoundString.plus(another: CompoundString): CompoundString =
    CompoundString(this.strings + another.strings)

LocalizedStringExt.kt

package ...

import android.content.Context

operator fun LocalizedString.get(context: Context) = unwrap(context)

fun LocalizedString.unwrap(
    context: Context
): CharSequence = when (this) {
    is Wrapped -> stringValue
    is CompoundString -> strings.joinToString(separator = "") { it.unwrap(context) }
    is ResourceString -> map(
        getStringFromResources(
            context,
            id,
            quantity,
            args.map { if (it is LocalizedString) it.unwrap(context) else it }
        )
    )
}

private fun getStringFromResources(
    context: Context,
    id: Int,
    quantity: Int?,
    args: List<Any>
): String = if (quantity == null) {
    context.getString(id, *args.toTypedArray())
} else {
    context.resources.getQuantityString(id, quantity, *args.toTypedArray())
}

"TypedString" sounds better

Create some pre-defined mappers, e.g. capitalzie, decapitalize