I have a font that supports only one language, when using the font with English I get the default English font for the language. I want to be able to add a specific English font to my language font. Is it possible ?
If you need the merged font in Adobe InDesign, you can do it directly from there, and it's quite flexible, as well.
Go to Type -> Composite Fonts, in the Composite Fonts Editor you see you can have three different fonts merged into one, one for your original language (Base), one for the English characters (Latin) and then you can choose what font you'd like for the numbers. Select any of the three sets and, on the bottom of the dialog, you can customize each set's size, baseline, vertical scale, and horizontal scale.
But wait, that's not all, you can actually have as many fonts merged together as you'd like, click on the Custom button and have a field day customizing whichever character you'd like to any font installed on your system and create a custom set, which you can then add to the original three sets, and then customize all aspects of each set on its own.