instead of CSS you can use a simple dom-if and avoid this problem. Both inputs (the fallback and the local) in the paper-local-input are showing up for me.
I DOn't know if it's due to Polymer 2, but paper-locale-input is showing both the fallback paper-input and the paper-locale-input. It seems like the properties are not passed through. Any plans to upgrade it to work with Polymer 2?
For non-numeric locale (e.g. Bengali where 1234567.toLocaleString('bn-BD') = ১২,৩৪,৫৬৭), allow to switch between an edit mode where we would input numeric values (when the field does not have the focus) and then back to the locale string value when paper-locale-input lose focus.