Cortana can handle most languages...

 Not right now but in the future. She probably does better than Siri in the number of languages with voice commands, but if web search is the mode she can handle a lot more languages than Siri.

Well first of all language settings for Cortana is supported in couple of countries like UK, China, France, Italy, Germany, Spain and perhaps few other countries. Let us say you travel from US to France and you want to find things there, using Cortana. Then you can switch the language that Cortana understands by default (US English in USA) to French. I assume you can access Cortana on your Windows 10 Desktop, If not go here.

In Windows 10 Desktop you can open Control Panel and choose to change your language preferences.

CortanaLang_01

In the above, I have added couple of languages ( this means I have downloaded the necessary language packs) and I have also set the default as French. You can add another language by hitting the Add a Language button. You may have to download the language pack.

With just this setting my Cortana settings in Windows 10 desktop is as shown.

CortanaLang_02

 Of course you cannot use it in USA and you will have to be in France.

Now I move up Japanese to the top in 'Change your language preferences'  as in the following:

CortanaLang_03

Voila! Cortana language settings has changed as well.

CortanaLang_04

You return to USA and you want to restore. It is just as easy. Just bump up English(United States) to the top (you need not restart the computer).

How about voice commands to Cortana? How about in Windows Phone? Well you will have to work with speech and speech recognition settings.

Come back again to learn, How.

Enabling Multilingual App Toolkit for Windows 8.1 Apps

Windows Phones are used worldwide and multilingual support is mandatory. The toolkit provides this much needed feature.

This toolkit integrates with Microsoft Visual Studio Toolkit to provide translation support
for Windows 8 Apps development and editor tools.

http://msdn.microsoft.com/en-us/library/windows/apps/hh465006.aspx

The toolkit provides (from MSDN resources) the following:

• Helps you manage resource changes and translation status during development.
• Provides a UI for choosing languages based on configured translation providers.
• Supports the localization industry-standard XLIFF file format.
• Provides a pseudo-language engine to help identify translation issues during development.
• Connects with the Microsoft Language Portal to easily access translated strings and terminology.
• Connects with the Microsoft Translator for quick translation suggestions.

You can download the toolkit for 11 different languages (language of the toolkit msi):

http://msdn.microsoft.com/en-us/windows/apps/bg127574

Once this is installed you should enable it in Visual Studio 2013 (works in Express 2013 with Update 2 RC).
According to MSDN it only becomes effective for projects created after it is installed which was slightly at variance.

A universal project developed with Windows App Studio beta was already opened in the VS IDE and when the multilingual toolkit was enabled it was enabled  only for the Windows Phone 8.1 part of the project and not the Windows 8.1.

For a blank hub project started in Visual Studio 2013, enabling  the Multilingual toolkit applies the
toolkit for both programs (Windows 8.1 and Windows Phone 8.1).

More details are posted here:
http://hodentekmobile.blogspot.com/2014/04/multilingual-support-for-windows-phone.html