You probably have just changed the *preferred* install location (I'm not aware of a setting that *enforces* a location for app install). So IMHO you don't need worry, simply install that app.

### Background:
Developers must declare their app's preferred install location in the `Manifest` of the app. There they basically have 3 choices: internal storage, external storage, let-the-OS-decide. On the OS side, again there's a corresponding setting: internal, external, default (usually internal). You altered the latter to "external" I assume.

Now what happens when you've set the OS side to "external"? Depends on what the `Manifest` of the app-to-be-installed says: if it says "let-the-OS-decide", things are clear: the app goes there (in your case, the external storage AKA SD card). Everything else overrides the OS-side setting: If an app offers services/widgets, it *must* go to internal storage, hence the developer will declare such – and the OS will obey.

So: No need to change anything. If the app *must* be installed to internal storage, the dev should have taken care for that in the app's `Manifest`. If you're still running into trouble, see [my answer here](https://android.stackexchange.com/a/43522/16575) on how to change the default.