ADDITIONAL SYSTEM INFORMATION :
System: Kubuntu 19.04 (Kernel 5.0.0-13-generic)

Working JDK ("./jdk1.8.0_211/bin/java -XshowSettings:locale -version"):
Locale settings:
    default locale = Deutsch
    default display locale = Deutsch (Deutschland)
    default format locale = Deutsch (Deutschland)
    available locales = , ar, 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, be, be_BY, bg, bg_BG, ca,
        ca_ES, cs, cs_CZ, da, da_DK, de, de_AT, de_CH,
        de_DE, de_GR, de_LU, el, el_CY, el_GR, en, en_AU,
        en_CA, en_GB, en_IE, en_IN, en_MT, en_NZ, en_PH, en_SG,
        en_US, en_ZA, es, 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, et_EE, fi, fi_FI, fr, fr_BE, fr_CA, fr_CH,
        fr_FR, fr_LU, ga, ga_IE, hi, hi_IN, hr, hr_HR,
        hu, hu_HU, in, in_ID, is, is_IS, it, it_CH,
        it_IT, iw, iw_IL, ja, ja_JP, ja_JP_JP_#u-ca-japanese, ko, ko_KR,
        lt, lt_LT, lv, lv_LV, mk, mk_MK, ms, ms_MY,
        mt, mt_MT, nl, nl_BE, nl_NL, no, no_NO, no_NO_NY,
        pl, pl_PL, pt, pt_BR, pt_PT, ro, ro_RO, ru,
        ru_RU, sk, sk_SK, sl, sl_SI, sq, sq_AL, sr,
        sr_BA, sr_BA_#Latn, sr_CS, sr_ME, sr_ME_#Latn, sr_RS, sr_RS_#Latn, sr__#Latn,
        sv, sv_SE, th, th_TH, th_TH_TH_#u-nu-thai, tr, tr_TR, uk,
        uk_UA, vi, vi_VN, zh, zh_CN, zh_HK, zh_SG, zh_TW
        
java version "1.8.0_211"
Java(TM) SE Runtime Environment (build 1.8.0_211-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.211-b12, mixed mode)


Not working JDK ("./jdk-12.0.1/bin/java -XshowSettings:locale -version"):
Locale settings:
    default locale = Deutsch (Deutschland)
    default display locale = Deutsch (Deutschland)
    default format locale = Deutsch (Deutschland)
    available locales = , af, af_NA, af_ZA, agq, agq_CM, ak, ak_GH,
        am, am_ET, ar, ar_001, ar_AE, ar_BH, ar_DJ, ar_DZ,
        ar_EG, ar_EH, ar_ER, ar_IL, ar_IQ, ar_JO, ar_KM, ar_KW,
        ar_LB, ar_LY, ar_MA, ar_MR, ar_OM, ar_PS, ar_QA, ar_SA,
        ar_SD, ar_SO, ar_SS, ar_SY, ar_TD, ar_TN, ar_YE, as,
        as_IN, asa, asa_TZ, ast, ast_ES, az, az_AZ_#Cyrl, az_AZ_#Latn,
        az__#Cyrl, az__#Latn, bas, bas_CM, be, be_BY, bem, bem_ZM,
        bez, bez_TZ, bg, bg_BG, bm, bm_ML, bn, bn_BD,
        bn_IN, bo, bo_CN, bo_IN, br, br_FR, brx, brx_IN,
        bs, bs_BA_#Cyrl, bs_BA_#Latn, bs__#Cyrl, bs__#Latn, ca, ca_AD, ca_ES,
        ca_ES_VALENCIA, ca_FR, ca_IT, ccp, ccp_BD, ccp_IN, ce, ce_RU,
        cgg, cgg_UG, chr, chr_US, ckb, ckb_IQ, ckb_IR, cs,
        cs_CZ, cu, cu_RU, cy, cy_GB, da, da_DK, da_GL,
        dav, dav_KE, de, de_AT, de_BE, de_CH, de_DE, de_IT,
        de_LI, de_LU, dje, dje_NE, dsb, dsb_DE, dua, dua_CM,
        dyo, dyo_SN, dz, dz_BT, ebu, ebu_KE, ee, ee_GH,
        ee_TG, el, el_CY, el_GR, en, en_001, en_150, en_AG,
        en_AI, en_AS, en_AT, en_AU, en_BB, en_BE, en_BI, en_BM,
        en_BS, en_BW, en_BZ, en_CA, en_CC, en_CH, en_CK, en_CM,
        en_CX, en_CY, en_DE, en_DG, en_DK, en_DM, en_ER, en_FI,
        en_FJ, en_FK, en_FM, en_GB, en_GD, en_GG, en_GH, en_GI,
        en_GM, en_GU, en_GY, en_HK, en_IE, en_IL, en_IM, en_IN,
        en_IO, en_JE, en_JM, en_KE, en_KI, en_KN, en_KY, en_LC,
        en_LR, en_LS, en_MG, en_MH, en_MO, en_MP, en_MS, en_MT,
        en_MU, en_MW, en_MY, en_NA, en_NF, en_NG, en_NL, en_NR,
        en_NU, en_NZ, en_PG, en_PH, en_PK, en_PN, en_PR, en_PW,
        en_RW, en_SB, en_SC, en_SD, en_SE, en_SG, en_SH, en_SI,
        en_SL, en_SS, en_SX, en_SZ, en_TC, en_TK, en_TO, en_TT,
        en_TV, en_TZ, en_UG, en_UM, en_US, en_US_POSIX, en_VC, en_VG,
        en_VI, en_VU, en_WS, en_ZA, en_ZM, en_ZW, eo, eo_001,
        es, es_419, es_AR, es_BO, es_BR, es_BZ, es_CL, es_CO,
        es_CR, es_CU, es_DO, es_EA, es_EC, es_ES, es_GQ, es_GT,
        es_HN, es_IC, es_MX, es_NI, es_PA, es_PE, es_PH, es_PR,
        es_PY, es_SV, es_US, es_UY, es_VE, et, et_EE, eu,
        eu_ES, ewo, ewo_CM, fa, fa_AF, fa_IR, ff, ff_CM,
        ff_GN, ff_MR, ff_SN, fi, fi_FI, fil, fil_PH, fo,
        fo_DK, fo_FO, fr, fr_BE, fr_BF, fr_BI, fr_BJ, fr_BL,
        fr_CA, fr_CD, fr_CF, fr_CG, fr_CH, fr_CI, fr_CM, fr_DJ,
        fr_DZ, fr_FR, fr_GA, fr_GF, fr_GN, fr_GP, fr_GQ, fr_HT,
        fr_KM, fr_LU, fr_MA, fr_MC, fr_MF, fr_MG, fr_ML, fr_MQ,
        fr_MR, fr_MU, fr_NC, fr_NE, fr_PF, fr_PM, fr_RE, fr_RW,
        fr_SC, fr_SN, fr_SY, fr_TD, fr_TG, fr_TN, fr_VU, fr_WF,
        fr_YT, fur, fur_IT, fy, fy_NL, ga, ga_IE, gd,
        gd_GB, gl, gl_ES, gsw, gsw_CH, gsw_FR, gsw_LI, gu,
        gu_IN, guz, guz_KE, gv, gv_IM, ha, ha_GH, ha_NE,
        ha_NG, haw, haw_US, hi, hi_IN, hr, hr_BA, hr_HR,
        hsb, hsb_DE, hu, hu_HU, hy, hy_AM, ig, ig_NG,
        ii, ii_CN, in, in_ID, is, is_IS, it, it_CH,
        it_IT, it_SM, it_VA, iw, iw_IL, ja, ja_JP, ja_JP_JP_#u-ca-japanese,
        jgo, jgo_CM, ji, ji_001, jmc, jmc_TZ, ka, ka_GE,
        kab, kab_DZ, kam, kam_KE, kde, kde_TZ, kea, kea_CV,
        khq, khq_ML, ki, ki_KE, kk, kk_KZ, kkj, kkj_CM,
        kl, kl_GL, kln, kln_KE, km, km_KH, kn, kn_IN,
        ko, ko_KP, ko_KR, kok, kok_IN, ks, ks_IN, ksb,
        ksb_TZ, ksf, ksf_CM, ksh, ksh_DE, kw, kw_GB, ky,
        ky_KG, lag, lag_TZ, lb, lb_LU, lg, lg_UG, lkt,
        lkt_US, ln, ln_AO, ln_CD, ln_CF, ln_CG, lo, lo_LA,
        lrc, lrc_IQ, lrc_IR, lt, lt_LT, lu, lu_CD, luo,
        luo_KE, luy, luy_KE, lv, lv_LV, mas, mas_KE, mas_TZ,
        mer, mer_KE, mfe, mfe_MU, mg, mg_MG, mgh, mgh_MZ,
        mgo, mgo_CM, mk, mk_MK, ml, ml_IN, mn, mn_MN,
        mr, mr_IN, ms, ms_BN, ms_MY, ms_SG, mt, mt_MT,
        mua, mua_CM, my, my_MM, mzn, mzn_IR, naq, naq_NA,
        nb, nb_NO, nb_SJ, nd, nd_ZW, nds, nds_DE, nds_NL,
        ne, ne_IN, ne_NP, nl, nl_AW, nl_BE, nl_BQ, nl_CW,
        nl_NL, nl_SR, nl_SX, nmg, nmg_CM, nn, nn_NO, nnh,
        nnh_CM, no, no_NO, no_NO_NY, nus, nus_SS, nyn, nyn_UG,
        om, om_ET, om_KE, or, or_IN, os, os_GE, os_RU,
        pa, pa_IN_#Guru, pa_PK_#Arab, pa__#Arab, pa__#Guru, pl, pl_PL, prg,
        prg_001, ps, ps_AF, pt, pt_AO, pt_BR, pt_CH, pt_CV,
        pt_GQ, pt_GW, pt_LU, pt_MO, pt_MZ, pt_PT, pt_ST, pt_TL,
        qu, qu_BO, qu_EC, qu_PE, rm, rm_CH, rn, rn_BI,
        ro, ro_MD, ro_RO, rof, rof_TZ, ru, ru_BY, ru_KG,
        ru_KZ, ru_MD, ru_RU, ru_UA, rw, rw_RW, rwk, rwk_TZ,
        sah, sah_RU, saq, saq_KE, sbp, sbp_TZ, sd, sd_PK,
        se, se_FI, se_NO, se_SE, seh, seh_MZ, ses, ses_ML,
        sg, sg_CF, shi, shi_MA_#Latn, shi_MA_#Tfng, shi__#Latn, shi__#Tfng, si,
        si_LK, sk, sk_SK, sl, sl_SI, smn, smn_FI, sn,
        sn_ZW, so, so_DJ, so_ET, so_KE, so_SO, sq, sq_AL,
        sq_MK, sq_XK, sr, sr_BA, sr_BA_#Cyrl, sr_BA_#Latn, sr_CS, sr_ME,
        sr_ME_#Cyrl, sr_ME_#Latn, sr_RS, sr_RS_#Cyrl, sr_RS_#Latn, sr_XK_#Cyrl, sr_XK_#Latn, sr__#Cyrl,
        sr__#Latn, sv, sv_AX, sv_FI, sv_SE, sw, sw_CD, sw_KE,
        sw_TZ, sw_UG, ta, ta_IN, ta_LK, ta_MY, ta_SG, te,
        te_IN, teo, teo_KE, teo_UG, tg, tg_TJ, th, th_TH,
        th_TH_TH_#u-nu-thai, ti, ti_ER, ti_ET, tk, tk_TM, to, to_TO,
        tr, tr_CY, tr_TR, tt, tt_RU, twq, twq_NE, tzm,
        tzm_MA, ug, ug_CN, uk, uk_UA, ur, ur_IN, ur_PK,
        uz, uz_AF_#Arab, uz_UZ_#Cyrl, uz_UZ_#Latn, uz__#Arab, uz__#Cyrl, uz__#Latn, vai,
        vai_LR_#Latn, vai_LR_#Vaii, vai__#Latn, vai__#Vaii, vi, vi_VN, vo, vo_001,
        vun, vun_TZ, wae, wae_CH, wo, wo_SN, xog, xog_UG,
        yav, yav_CM, yo, yo_BJ, yo_NG, yue, yue_CN_#Hans, yue_HK_#Hant,
        yue__#Hans, yue__#Hant, zgh, zgh_MA, zh, zh_CN, zh_CN_#Hans, zh_HK,
        zh_HK_#Hans, zh_HK_#Hant, zh_MO_#Hans, zh_MO_#Hant, zh_SG, zh_SG_#Hans, zh_TW, zh_TW_#Hant,
        zh__#Hans, zh__#Hant, zu, zu_ZA
openjdk version "12.0.1" 2019-04-16
OpenJDK Runtime Environment (build 12.0.1+12)
OpenJDK 64-Bit Server VM (build 12.0.1+12, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
Swing dialogs are displayed in English instead of the system language. In Java 1.8 it worked as expected, with the same source code.

REGRESSION : Last worked in version 8u211

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute the source code below

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Buttons are labeled in system language (in German: "Ja" and "Nein"), like it does under Java 1.8
ACTUAL -
Buttons are labeled in English ("Yes" and "No")

---------- BEGIN SOURCE ----------
import javax.swing.JOptionPane;

public class SwingTest {
public static void main(String[] args) {
JOptionPane.showConfirmDialog(null, "Test", "Test", JOptionPane.YES_NO_OPTION);
}
}
---------- END SOURCE ----------

FREQUENCY : always