Sample code for getting first day data in Java:

for(Locale target:Calendar.getInstance().getAvailableLocales()){
    Calendar cal = Calendar.getInstance(target);
    int javaResult = cal.getFirstDayOfWeek();
        }

Comparing to CLDR result in supplemntalData.xml, there are 5 locales are different:
Locale is: bs-BA, Country is: BA
--First day of week in JAVA is: 1
--First day of week in CLDR is: mon(2)
--------------
Locale is: uz-AF, Country is: AF
--First day of week in JAVA is: 1
--First day of week in CLDR is: sat(7)
--------------
Locale is: az-AZ, Country is: AZ
--First day of week in JAVA is: 1
--First day of week in CLDR is: mon(2)
--------------
Locale is: shi-MA, Country is: MA
--First day of week in JAVA is: 1
--First day of week in CLDR is: sat(7)
--------------
Locale is: uz-UZ, Country is: UZ
--First day of week in JAVA is: 1
--First day of week in CLDR is: mon(2)
--------------

Supplemental data:
<firstDay day="mon" territories="
001
AD AI AL AM AN AT AX AZ
BA BE BG BM BN BY
CH CL CM CR CY CZ
DE DK
EC EE ES
FI FJ FO FR
GB GE GF GP GR
HR HU
IS IT
KG KZ
LB LI LK LT LU LV
MC MD ME MK MN MQ MY
NL NO
PL PT
RE RO RS RU
SE SI SK SM
TJ TM TR
UA UY UZ
VA VN
XK"
/>
          <firstDay day="fri" territories="BD MV" />
          <firstDay day="sat" territories="AE AF BH DJ DZ EG IQ IR JO KW LY MA OM QA SD SY" />
          <firstDay day="sun" territories="
AG AR AS AU
BR BS BT BW BZ
CA CN CO
DM DO
ET
GT GU
HK HN
ID IE IL IN
JM JP
KE KH KR
LA
MH MM MO MT MX MZ
NI NP NZ
PA PE PH PK PR PY
SA SG SV
TH TN TT TW
UM US
VE VI
WS
YE
ZA ZW"
          />