CLANDESTINELY

Chinese 

秘密地 (secretly). (various references)

Dutch

tersluiks (stealthily), sluiks (stealthily), in bedekte termen (secretly), heimelýk (secret, secretly, veiled). (various references)

Esperanto

kaŝe (secretly). (various references)

French

clandestinement, clandestine. (various references)

German

heimliche (covertly, stealthy, underhandedly), heimlich (clandestine, cryptical, secret, stealthy). (various references)

Hebrew 

בלאט (secretly). (various references)

Hungarian

rejtetten (criptically, occultly), alattomban (on the sly, surreptitiously, underhand). (various references)

Korean 

은밀하게 (Confidentially, Covertly, furtively, secretly). (various references)

Manx

gyn yss (by stealth, covert, in secret, on the sly, secretly, surreptitious, surreptitiously, unadvised, unawares, unconsciously, unexpected). (various references)

Pig Latin

andestinelyclay.(various references)

Romanian

clandestin (clandestine, furtive, furtively, hugger mugger, illicit, privy, secret, secretly, surreptitious, surreptitiously, underhand). (various references)

Spanish

clandestinamente (stealthily, underground). (various references)

Turkish

gizlice (by devious means, by stealth, confidentially, furtively, in privacy, in private, in secret, on the quiet, on the sly, secretly, sub rosa, under the counter, under the rose, underground), el altından (by devious means, by stealth, clandestine, hole-and-corner, in secret, on the sly, secretly, slily, sub rosa, surreptitious, under the counter, under the rose, undercover, underhand, underhanded). (various references)