I add languages in Windows 10 v1607 like this:

$list = Get-WinUserLanguageList
$list.Add("nl-BE")
Set-WinUserLanguageList -LanguageList $list

Windows 10 thinks I have this list of languages currently installed (the result of a Get-WinUserLanguageList command):
LanguageTag : nl-BE
Autonym : Nederlands (België)
EnglishName : Dutch
LocalizedName : Dutch (Belgium)
ScriptName : Latin script
InputMethodTips : {0813:00000813}
Spellchecking : True
Handwriting : False

LanguageTag : en-US
Autonym : English (United States)
EnglishName : English
LocalizedName : English (United States)
ScriptName : Latin script
InputMethodTips : {0409:00000409}
Spellchecking : True
Handwriting : False

LanguageTag : de-DE
Autonym : Deutsch (Deutschland)
EnglishName : German
LocalizedName : German (Germany)
ScriptName : Latin script
InputMethodTips : {0407:00000407}
Spellchecking : True
Handwriting : False

LanguageTag : fr-FR
Autonym : français (France)
EnglishName : French
LocalizedName : French (France)
ScriptName : Latin script
InputMethodTips : {040C:0000040C}
Spellchecking : True
Handwriting : False

LanguageTag : it-IT
Autonym : italiano (Italia)
EnglishName : Italian
LocalizedName : Italian (Italy)
ScriptName : Latin script
InputMethodTips : {0410:00000410}
Spellchecking : True
Handwriting : False

LanguageTag : ru
Autonym : ???????
EnglishName : Russian
LocalizedName : Russian
ScriptName : Cyrillic
InputMethodTips : {0419:00000419}
Spellchecking : True
Handwriting : False

LanguageTag : es-ES
Autonym : Español (España)
EnglishName : Spanish
LocalizedName : Spanish (Spain)
ScriptName : Latin script
InputMethodTips : {0C0A:0000040A}
Spellchecking : True
Handwriting : False

LanguageTag : pl
Autonym : polski
EnglishName : Polish
LocalizedName : Polish
ScriptName : Latin script
InputMethodTips : {0415:00000415}
Spellchecking : True
Handwriting : False

LanguageTag : pt-BR
Autonym : Português (Brasil)
EnglishName : Portuguese
LocalizedName : Portuguese (Brazil)
ScriptName : Latin script
InputMethodTips : {0416:00000416}
Spellchecking : True
Handwriting : False

LanguageTag : zh-Hans-CN
Autonym : ??(???????)
EnglishName : Chinese
LocalizedName : Chinese (Simplified, China)
ScriptName : Chinese (Simplified)
InputMethodTips : {0804:{81D4E9C9-1D3B-41BC-9E6C-4B40BF79E35E}{FA550B04-5AD7-411F-A5AC-CA038EC515D
7}}
Spellchecking : True
Handwriting : True

LanguageTag : ja
Autonym : ???
EnglishName : Japanese
LocalizedName : Japanese
ScriptName : Japanese
InputMethodTips : {0411:{03B5835F-F03C-411B-9CE2-AA23E1171E36}{A76C93D9-5523-4E90-AAFA-4DB112F9AC7
6}}
Spellchecking : True
Handwriting : True

LanguageTag : ko
Autonym : ???
EnglishName : Korean
LocalizedName : Korean
ScriptName : Korean
InputMethodTips : {0412:{A028AE76-01B1-46C2-99C4-ACD9858AE02F}{B5FE1F02-D5F2-4445-9C03-C568F23C99A
1}}
Spellchecking : True
Handwriting : True

LanguageTag : hu
Autonym : magyar
EnglishName : Hungarian
LocalizedName : Hungarian
ScriptName : Latin script
InputMethodTips : {040E:0000040E}
Spellchecking : True
Handwriting : False

LanguageTag : fi
Autonym : suomi
EnglishName : Finnish
LocalizedName : Finnish
ScriptName : Latin script
InputMethodTips : {040B:0000040B}
Spellchecking : True
Handwriting : False

LanguageTag : sv-SE
Autonym : svenska (Sverige)
EnglishName : Swedish
LocalizedName : Swedish (Sweden)
ScriptName : Latin script
InputMethodTips : {041D:0000041D}
Spellchecking : True
Handwriting : False

LanguageTag : tr
Autonym : Türkçe
EnglishName : Turkish
LocalizedName : Turkish
ScriptName : Latin script
InputMethodTips : {041F:0000041F}
Spellchecking : True
Handwriting : False

LanguageTag : cs
Autonym : ceština
EnglishName : Czech
LocalizedName : Czech
ScriptName : Latin script
InputMethodTips : {0405:00000405}
Spellchecking : True
Handwriting : False

LanguageTag : zh-Hant-TW
Autonym : ??(??)
EnglishName : Chinese
LocalizedName : Chinese (Traditional, Taiwan)
ScriptName : Chinese (Traditional)
InputMethodTips : {0404:{B115690A-EA02-48D5-A231-E3578D2FDF80}{B2F9C502-1742-11D4-9790-0080C882687
E}}
Spellchecking : True
Handwriting : True

LanguageTag : nl-NL
Autonym : Nederlands (Nederland)
EnglishName : Dutch
LocalizedName : Dutch (Netherlands)
ScriptName : Latin script
InputMethodTips : {0413:00020409}
Spellchecking : True
Handwriting : False

But then the Virtual Keyboard Layout Creator shows this list:
English (United States)
Finnish (Finland)
French (France)
Hungarian (Hungary)
Italian (Italy)
Japanese (Japan)
Korean (Korea)
Serbian (Latin, Serbia and Montenegro (Former))
Polish (Poland)
Portuguese (Brazil)
Russian (Russia)
Swedish (Sweden)
German (Germany)
Czech (Czech Republic)
Chinese (Traditional, Taiwan)
Dutch (Belgium)

So the VKLC is missing these:
LanguageTag : es-ES
Autonym : Español (España)
EnglishName : Spanish
LocalizedName : Spanish (Spain)
ScriptName : Latin script
InputMethodTips : {0C0A:0000040A}
Spellchecking : True
Handwriting : False

LanguageTag : zh-Hans-CN
Autonym : ??(???????)
EnglishName : Chinese
LocalizedName : Chinese (Simplified, China)
ScriptName : Chinese (Simplified)
InputMethodTips : {0804:{81D4E9C9-1D3B-41BC-9E6C-4B40BF79E35E}{FA550B04-5AD7-411F-A5AC-CA038EC515D
7}}
Spellchecking : True
Handwriting : True


LanguageTag : tr
Autonym : Türkçe
EnglishName : Turkish
LocalizedName : Turkish
ScriptName : Latin script
InputMethodTips : {041F:0000041F}
Spellchecking : True
Handwriting : False


LanguageTag : nl-NL
Autonym : Nederlands (Nederland)
EnglishName : Dutch
LocalizedName : Dutch (Netherlands)
ScriptName : Latin script
InputMethodTips : {0413:00020409}
Spellchecking : True
Handwriting : False

And it's showing an extra one:
Serbian

Interestingly, I can insert languages to the TOP of the language list (with powershell as described above) and then I can see them but others drop off the list exposed by VKLC.

This build should show all languages -
https://mindfusion.eu/_beta/VirtualKeyboardCreator.zip

Regards,
Slavcho