Question: How can we test K4 to see if it’s a cipher of a different language?

Observations of K4
It has withstood almost 20 years of cryptanalysis
The letter frequency is not English plaintext
The letter frequency is muted
Ed Scheidt revealed that he used a form of masking or steganography
Twist and turn it all you want, K4 is no good as is
We don’t know what clues they left us for K4 or what they mean
We don’t even know how we were supposed to know how to solve K1-3 besides brute force

Hypothesis: K4 was written in English, translated into another language and then ciphered.

Assumptions/Tests
The language would be of Latin origin, one of the Romance languages
It is probably a commonly used language, i.e. not an old dialect of gypsies in Ukraine
Most likely is German (WWI/WWII) then French, Spanish, Portuguese and Italian based on similarities to English
Probably not Russian
If letters do not exist in the language then they will be regarded as nulls.
Substitution will be considered more likely than transposition
Special preference will be given to “named” ciphers of WWI and WWII
It is likely that all of the ciphertext will become plaintext
Possible that one set of letters (A-Z) has been added to confuse the issue

Methods
Compare letter frequencies across languages and K4
Pursue any possible leads for transposition
Consider higher frequency letters for a substitution in each language
Organize a list of WWI/WWII commonly used pencil/paper ciphers, i.e. no cipher machines
Try/try/try and then give up

I plan to start by using some known language letter frequencies.

For a quick order of those letter frequencies in an easier layout: the most common letters.

For a manual cryptogram solver that will save countless trees by automatically showing the substitutions: Mark’s Cryptogram Solution Aid

The languages I’ve attempted:

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English

German

Spanish

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