The naming convention for SubRip (.srt) files is:
filename.[language code]_[country code].srt

Language and country codes are case sensitive. Language needs to be in lower case and country needs to be in upper case. For example, a United States English file would look like: filename.en_US.srt.
Listed below are valid language and country code combinations.

af_ZA (Afrikaans)
ar_AR (Arabic)
ay_BO (Aymara)
az_AZ (Azerbaijani)
be_BY (Belarusian)
bg_BG (Bulgarian)
bn_IN (Bengali)
bs_BA (Bosnian)
ca_ES (Catalan)
ck_US (Cherokee)
cs_CZ (Czech)
cx_PH (Cebuano)
cy_GB (Welsh)
da_DK (Danish)
de_DE (German)
el_GR (Greek)
en_GB (English - UK)
en_IN (English - India)
en_US (English - US)
eo_EO (Esperanto)
es_CL (Spanish - Chile)
es_CO (Spanish - Colombia)
es_ES (Spanish - Spain)
es_LA (Spanish)
es_MX (Spanish - Mexico)
es_VE (Spanish - Venezuela)
et_EE (Estonian)
eu_ES (Basque)
fa_IR (Persian)
fi_FI (Finnish)
fo_FO (Faroese)
fr_CA (French - Canada)
fr_FR (French - France)
fy_NL (Frisian)
ga_IE (Irish)
gl_ES (Galician)
gn_PY (Guarani)
gu_IN (Gujarati)
gx_GR (Classical Greek)
he_IL (Hebrew)
hi_IN (Hindi)
hr_HR (Croatian)
hu_HU (Hungarian)
hy_AM (Armenian)
id_ID (Indonesian)
is_IS (Icelandic)
it_IT (Italian)
ja_JP (Japanese)
jv_ID (Javanese)
ka_GE (Georgian)
kk_KZ (Kazakh)
km_KH (Khmer)
kn_IN (Kannada)
ko_KR (Korean)
ku_TR (Kurdish)
la_VA (Latin)
li_NL (Limburgish)
lt_LT (Lithuanian)
lv_LV (Latvian)
mg_MG (Malagasy)
mk_MK (Macedonian)
ml_IN (Malayalam)
mn_MN (Mongolian)
mr_IN (Marathi)
ms_MY (Malay)
mt_MT (Maltese)
nb_NO (Norwegian - Bokmal)
ne_NP (Nepali)
nl_BE (Dutch - Belgium)
nl_NL (Dutch)
nn_NO (Norwegian - Nynorsk)
pa_IN (Punjabi)
pl_PL (Polish)
ps_AF (Pashto)
pt_BR (Portuguese - Brazil)
pt_PT (Portuguese - Portugal)
qu_PE (Quechua)
rm_CH (Romansh)
ro_RO (Romanian)
ru_RU (Russian)
sa_IN (Sanskrit)
se_NO (Northern Sami)
sk_SK (Slovak)
sl_SI (Slovenian)
so_SO (Somali)
sq_AL (Albanian)
sr_RS (Serbian)
sv_SE (Swedish)
sw_KE (Swahili)
sy_SY (Syriac)
ta_IN (Tamil)
te_IN (Telugu)
tg_TJ (Tajik)
th_TH (Thai)
tl_PH (Filipino)
tr_TR (Turkish)
tt_RU (Tatar)
uk_UA (Ukrainian)
ur_PK (Urdu)
uz_UZ (Uzbek)
vi_VN (Vietnamese)
xh_ZA (Xhosa)
yi_DE (Yiddish)
zh_CN (Simplified Chinese - China)
zh_HK (Traditional Chinese - Hong Kong)
zh_TW (Traditional Chinese - Taiwan)
zu_ZA (Zulu)