[TrackerFF]

Likewise, in the military, the use of countersigns have been designed to make non-native speakers stand out - should the countersign be compromised. For example, in WW2, Americans would use "Lollapalooza", as Japanese really struggled with that word.

[PeterisP]

That's more of a shibboleth than a secret, which is literally a practice as old as the Bible - "And the Gileadites took the passages of Jordan before the Ephraimites: and it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay; Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. Then they took him, and slew him at the passages of Jordan: and there fell at that time of the Ephraimites forty and two thousand."

[jhgb]

If you combine a shibboleth with a secret, how do you call it? Shibbecret?

[kragen]

Hmm, I'd think that in a rhotic accent a word like "furlstrengths" or "fatherlands" would work better? In Japanese they sound like [ɸɯ̟ɾɯ̟ɾɯ̟sɯ̟tɯ̟ɾiĩsɯ̟] or [haɾɯ̟sɯ̟tɯ̟ɾiĩsɯ̟] and [hazaɾɯ̟randozɯ̟] respectively, rather than the native [fɚɹłstɹiŋθs] or [fɚɹłstɹiŋkθs] and [faðɚlændz]. Adjacent /rl/ pairs are a special challenge, there are multiple unvoiced fricatives that don't exist at all in Japanese, and consonant clusters totally violate Japanese phonotactics to the point where it's hard for Japanese people to even detect the presence of some of the consonants. By contrast Japanese [ɾaɾapaɾɯ̟za] is only slightly wrong, requiring a little bit more bilateral bypass on the voiced taps and a slight rounding of the ɯ̟ sound.

Some Japanese-American soldiers would be SOL tho.