Preview As Visitor unusable if JVM default language is not plain english

Steps to reproduce

1.  Open https://demo.magnolia-cms.com/.magnolia/admincentral#app:preview-app:detail;/travel:view:
2.  Select a trait value

Expected results

Value is selected.

Actual results

Field is reset and empty.

Workaround

On some installations, second selection of the value works. Anyway, to workaround the issue:

1. set the JVM default locale to plain en (no country specified) -Duser.language=en -Duser.country=
2. To verify the parameters were passed to JVM correctly, go to about app https://demo.magnolia-cms.com/.magnolia/admincentral#app:about:config
3. Search for user.language, should be en
4. Search for user.country, either this property is not there or the value is empty

Development notes

• p13n preview has the locale hardcoded to en.
• p13n presenter delegates to deprecated ctor of FormPresenter which uses Locale.getDefault() as default language.
• In case these two differ (even different country like en_EN), two versions of form are created
• The en version communicates with the backend, but en_EN is visible