Data for webpages can and will come from many sources, External api and external databases are two very common sources but also some will need to be content managed. 

Having that source within the same Umbraco instance makes perfect sense. The issue is how to demarcate local data from global data.

Following Global Manifesto rules and keep the global data outside and away from local data.

There is no hard and fast rule on how to handle Multiple languages for global data, but a prefernce would be to create a single node to represent the single element of data and if multiple languages are requires, for descriptions etc, then Umbraco has a built-in Vary by Culture Feature that you allow the entry of multiple langauges for the same Property Editor

Its quite possible to have thousands, if not millions of global data records and sometimes the standard Tree view that Umbraco uses might not be the most relevant way to interact with this data. 

List View creates a more bucket type approach to your global data. Giving you access to your data via a page view coupled with a search feature