This is a complete nothing-post, I'm merely putting it here because I was searching for the list of values online (to save writing the code below, laziness eh?) and couldn't find them, so here they are, correct at the time of writing.

List<String> values = new List<String>();
Schema.SObjectField f = Schema.getGlobalDescribe()
                     .get('User')
                     .getDescribe()
                     .Fields.getMap()
                     .get('LocaleSidKey');

for (Schema.PicklistEntry picklistEntry : f.getDescribe().getPicklistValues())
{
	values.add(picklistEntry.getValue());
}
 
values.sort();
 
for(String s : values)
{
	System.Debug(s);
}
af_ZA
ar_AE
ar_BH
ar_DZ
ar_EG
ar_IQ
ar_JO
ar_KW
ar_LB
ar_LY
ar_MA
ar_OM
ar_QA
ar_SA
ar_SD
ar_SY
ar_TN
ar_YE
az_AZ
be_BY
bg_BG
bn_BD
bn_IN
bs_BA
ca_ES
cs_CZ
cy_GB
da_DK
de_AT
de_BE
de_CH
de_DE
de_LU
dz_BT
el_GR
en_AG
en_AU
en_BB
en_BM
en_BS
en_BW
en_BZ
en_CA
en_CM
en_ER
en_FJ
en_FK
en_GB
en_GH
en_GI
en_GM
en_GY
en_HK
en_ID
en_IE
en_IN
en_JM
en_KE
en_KY
en_LR
en_MG
en_MU
en_MW
en_MY
en_NA
en_NG
en_NZ
en_PG
en_PH
en_PK
en_RW
en_SB
en_SC
en_SG
en_SH
en_SL
en_SX
en_SZ
en_TO
en_TT
en_TZ
en_UG
en_US
en_VU
en_WS
en_ZA
es_AR
es_BO
es_CL
es_CO
es_CR
es_CU
es_DO
es_EC
es_ES
es_GT
es_HN
es_MX
es_NI
es_PA
es_PE
es_PR
es_PY
es_SV
es_US
es_UY
es_VE
et_EE
eu_ES
fa_IR
fi_FI
fr_BE
fr_CA
fr_CH
fr_FR
fr_GN
fr_HT
fr_KM
fr_LU
fr_MC
fr_MR
fr_WF
ga_IE
gu_IN
hi_IN
hr_HR
hu_HU
hy_AM
in_ID
is_IS
it_CH
it_IT
iw_IL
ja_JP
ka_GE
kk_KZ
km_KH
kn_IN
ko_KP
ko_KR
ky_KG
lb_LU
lo_LA
lt_LT
lu_CD
lv_LV
mi_NZ
mk_MK
ml_IN
mr_IN
ms_BN
ms_MY
mt_MT
my_MM
ne_NP
nl_AW
nl_BE
nl_NL
nl_SR
no_NO
pl_PL
ps_AF
pt_AO
pt_BR
pt_CV
pt_MZ
pt_PT
pt_ST
rm_CH
rn_BI
ro_MD
ro_RO
ru_KZ
ru_RU
sh_BA
sh_CS
sh_ME
sh_ME_USD
sk_SK
sl_SI
so_DJ
so_SO
sq_AL
sr_BA
sr_CS
sr_RS
sv_SE
sw_KE
ta_IN
ta_LK
te_IN
tg_TJ
th_TH
ti_ET
tl_PH
tr_TR
uk_UA
ur_PK
uz_LATN_UZ
vi_VN
xh_ZA
yo_BJ
zh_CN
zh_CN_PINYIN
zh_CN_STROKE
zh_HK
zh_HK_STROKE
zh_MO
zh_SG
zh_TW
zh_TW_STROKE
zu_ZA