You don’t need to use ENVI or Matlab. SNAP is all you need.
If you want special bands for latitude and longitude you can create them with Band Maths.
Create one band named latitude and use as expression LAT. Create a second band named longitude and use as expression LON.
When you export the data you can choose to export a subset or make a subset before you export it.
In the subset dialog you can choose the bands you want to include in the export.
The other option is to export a mask pixels. You can create a maks by drawing a rectangle or polygon around the area of interest or create one in the Mask Manager by using an expression.
Afterwards you can select the mask in the view and and then ricght-click in the view and choose from the context menu ‘Export Mask Pixels’. This will write a CSV file containing the data of your product including latitude and longitude. Be careful such files can easily get big.
So you see:
All you need is SNAP 
