How to add Dictation Languages

No you cannot add a dictation language beyond those provided by Apple. To ask Apple to provide the feature you want, go to

http://www.apple.com/feedback

Apple's publish specs:

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

You can add a Keyboard but not Dictionary

QuickType Keyboard: Predictive Input

• English (U.S.)
• English (Canada)
• English (UK)
• English (Australia)
• English (India)
• English (Singapore)
• Chinese (Simplified)
• Chinese (Traditional)
• French (France)
• French (Switzerland)
• French (Canada)
• French (Belgium)
• German (Germany)
• German (Austria)
• German (Switzerland)
• Italian
• Japanese
• Korean
• Spanish (Spain)
• Spanish (Latin America)
• Spanish (Mexico)
• Arabic (Modern Standard)
• Arabic (Najdi)
• Cantonese (Traditional)
• Dutch
• Hindi (Devanagari)
• Hindi (Latin)
• Portuguese (Portugal)
• Portuguese (Brazil)
• Russian
• Swedish
• Thai
• Turkish
• Vietnamese

You may search for third party apps on App Store