Microsoft is helping developers by merging the two main platforms (Windows & Phone), but unfortunately, if you do a search on Universal App you will get a lot of information about another company trying to combine their platforms.
In the second update to Visual Studio 2013, they added supported for Windows Phone 8.1 and Universal Apps. So far it is not a complete merger because Windows and the Phones still have differences in their capabilities and API calls, but they are heading towards a single development model. The Universal App model is a blend of Phone and Windows development, taking the best from each.
Here is a good link to get started.
http://Building universal Windows apps for all Windows devices
One good link on that page is to a set of samples:
Universal Windows app samples
Unfortunately, though there are short descriptions of each sample, it is hard to figure out the framework they use. It seems someone at Microsoft forgot that these are provided as learning examples and are trying to impress with cleverness. Sorry, that does not work. Writing simple and clear examples is the way to go.
The Universal Apps are created with three sections: Windows, Phone and Shared and the structure is easy to navigate. Though not implemented, adding the Multilingual Toolkit is just a matter of enabling it in Visual Studio and adding the appropriate languages.
So, it is now time to abandon phone development and concentrate on Universal Apps.
Here are some more links:
Using Visual Studio to build Universal XAML Apps
John Marshall… Visio MVP Visio.MVPs.org