• scala – encodeToString is not a member of Base64? – Stack Overflow
• convert a byte array to string – Stack Overflow
• How to convert byte array to string in Scala? (includehelp.com)
• java.nio.charset.StandardCharsets Scala Example (programcreek.com)

import java.util.Base64
import java.nio.charset.StandardCharsets

val plainText: String = "abcdefg"

val encoded: String = Base64.getEncoder.encodeToString(plainText.getBytes("UTF-8"))

val decoded: Array[Byte] = Base64.getDecoder.decode(encoded)

val str1 = new String(decoded)
val str2 = new String(decoded, StandardCharsets.UTF_8)

output

plainText: String = abcdefg
encoded: String = YWJjZGVmZw==
decoded: Array[Byte] = Array(97, 98, 99, 100, 101, 102, 103)
str1: String = abcdefg
str2: String = abcdefg