Thank you for your post.
I am unable to replicate the issue. This is what I have done.
1. Using a Mac OS X 10.11.5 iMac, I went into System Preferences and changed my Language and Region to Francais and France, respectively.
2. I launched FileMaker Pro Advanced 15.0.1, and all menus appear in French.
3. I created a new solution named 1z.fmp12 with a text field "Name" and a Date field "Date".
4. I entered one record with the data Name="TSGal" and Date=25/12/2016.
5. I quit FileMaker Pro.
6. In System Preferences, I changed Language and Region back to English and United States.
7. I opened 1z.fmp12 with FileMaker Pro Advanced 15.0.1, and the data appears as "TSGal" and 12/25/2016.
8. I saved a clone of the file with the name "1z Clone.fmp12".
9. I set the Admin password to "admin"
10. I set the file for network sharing and configured for WebDirect.
11. I uploaded the file to FileMaker Server 15 on both Windows Server 2012 R2 and Mac OS X 10.11.5.
12. On the Mac OS X 10.11.5 machine, I launched Safari 9.1.1 and opened the file via WebDirect.
13. I created a new record and entered Name="TSGal" and Date=12/25/2016.
14. I launched Chrome 51, and opened the file via WebDirect. Date still appears as 12/25/2016.
15. I entered another record with values Name="TSGal" and Date=25/12/2016, and I got the error message that the value must be a valid date.
Let me know what I am doing differently than you so I can replicate the issue.