Authorized users can add custom speech recognition languages (“Speech languages” page in the user account). Language codes must be constructed, according the bcp47 specification. For example for USA English this code is en-US
.
Be attention with the case of the letters.

You can hide predefined languages from drop-down list in the speechpad.pw page by pressing Hide predefined languages button. In this case, only your languages will be shown. The first added language will be selected when Voice notebook starts.

You can use voice commands Change language 1 and Change language 2 to select a next language from that truncated list (the next language after the last is first). For example, if we added two language: English and French, then we can use keyword “change language” for the command while dictating in English, and “changer la langue” if the French language is used. If the quantity of languages is greater than 2 you can use a number (for example 124) as the voice command word. If you spell this number in any of your languages – the change language command will be triggered.

You can add the parameter pagelang=YourLangCode to the query string to start Notebook with the desired language. If the language is added by the user, then the user must be logged into the site (must not to press log out when he quit the site). For example this link will open Voice notebook and set German language https://voicenotebook.com?pagelang=de-DE.

Below are the language codes that you can use (the same codes uses Notebook extension):

af-ZA          Afrikaans    
id-ID          Bahasa Indonesia    
ms-MY          Bahasa Melayu    
ca-ES          Català    
cs-CZ          Čeština    
da-DK          Dansk    
de-DE          Deutsch    
en-GB          English (United Kingdom)    
en-US          English (United States)    
es-ES          Español (España)    
es-419          Español (Latinoamérica)    
eu-ES          Euskara    
fil-PH          Filipino    
fr-FR          Français    
gl-ES          Galego    
hr-HR          hrvatski    
zu-ZA          IsiZulu    
is-IS          Íslenska    
it-IT          italiano    
lt-LT          Lietuvių    
hu-HU          Magyar    
nl-NL          Nederlands    
nb-NO          Norsk (Bokmål)    
pl-PL          Polski    
pt-BR          Português (Brasil)    
pt-PT          Português (Portugal)    
ro-RO          Română    
sl-SI          Slovenščina    
sk-SK          Slovenčina    
fi-FI          Suomi    
sv-SE          Svenska    
vi-VN          Tiếng Việt    
tr-TR          Türkçe    
el-GR          Ελληνικά    
bg-BG          български    
ru-RU          Pусский    
sr-RS          Српски    
uk-UA          Українська    
he-IL          עברית    
ar-x-gulf      العربية     
fa-IR          فارسی     
hi-IN          हिन्दी     
th-TH          ไทย     
cmn-Hans-CN    中文（中国）    
cmn-Hant-TW    中文（台灣）    
yue-Hant-HK    中文（香港）    
ja-JP          日本語    
ko-KR          한국어    

09.08.2016. Below are the language codes, that use Google Cloud Speech API. It seems to me that we can use them too (follow the cloud link to get up to date list. 30 new languages have been added).