How to localize the images in Images.xcassets?

That "Localize..." button is back in Xcode 11! So now you can localize your images and assets in the Asset catalog as you expect:

If you are using an Asset Catalog:

Asset catalog elements are now localizable. In the information panel for an asset, there is a "Localization" section that lists all the languages you've set up for your project in the project settings. If you don't select any of them, your images will be deemed "universal" (i.e., they will adopt the default behavior). If you select one or more of them, you will be able to select different images for "universal" and any alternate language.

For example, if your base language is English, and you want Spanish-language alternates, select only "Spanish" and drag in the alternate versions in the Spanish wells, and leave the English versions as the Universal. Then, at run-time, if the chosen language is Spanish, then the Spanish-language image will be pulled. Otherwise, it will pull the English version. (Or, if you want specific English and Spanish versions that are both different from "everything else", also check English and drag in the corresponding English and Universal images.)

If you need to determine localized images at run time without using the Asset Catalog:

While there's (apparently) not currently a way to explicitly localize the xcassets file directly, you could instead localize the name of the asset to pull using your Localizable.strings file. For instance, say you have a graphic logo treatment for the main menu of a game that needs to be localized. You could put the localized logo treatments in the assets file with different names, add the names to your Localizable.strings file, and then fetch the appropriate image with code like this:

UIImage *img = [UIImage imageNamed:NSLocalizedString(@"MAIN_MENU_IMAGE", nil)];