Hello, how to add a language to the "dictation language" function on iPhone 5s

You can't add any languages to the dictation function beyond what Apple provides already.

https://www.apple.com/ios/feature-availability/#quicktype-keyboard-dictation

To ask for more, go to

http://www.apple.com/feedback

Add a keyboard of the desired language and if the language is supported for dictation then dictation language is automatically added under Settings > General > > Keyboard > Dictation Language