Around and about one hundred years ago, your idea would surely have made sense… but nowadays, modern technology and evolved cryptanalytic techniques are too smart to have a real problem coping with something like that. (Also see my related answer to “Why was the Navajo code not broken by the Japanese in WWII?”)
Even when we completely ignore Kerckhoffs’ principle that “the enemy knows the system” (something we usually assume to be true in the realms of professional cryptography) the answer would still be a plain and simple “not really”.
As a matter a fact, you might as well write your whole thing in Navajo and mix in some Klingon before encrypting things… and it still wouldn’t buy you any free lunch in terms of
difficult to decrypt.
Tip: “multiple encryption” is a more solid alternative in terms of security.
If you really want to add some decent security, you might want to know that there is a much more constructive way to handle things: use multiple ciphers – practically encrypting your plaintext two or more times using different algorithms and/or different cipher modes.
Some (not all!) random examples to give you an idea of what I mean:
- using multiple ciphers:
plaintext $\rightarrow$ encrypt with AES block cipher $\rightarrow$ encrypt with ChaCha20 stream cipher
- or, using one cipher but multiple cipher modes of operation:
plaintext $\rightarrow$ encrypt with AES in GCM mode $\rightarrow$ encrypt with AES in CTR mode
- or, maybe even a combination of the above:
plaintext $\rightarrow$ encrypt with AES-CGM $\rightarrow$ encrypt with ChaCha20 $\rightarrow$ encrypt with AES-CTR
You could go crazy with this until you run out of time, resources, and/or combinations of modes and algorithms… but usually, multi-encrypting your plaintext with 2 different ciphers is more than enough.
As long as you avoid broken things like RC4 and as long as you encrypt things correctly (meaning: “by the book”, thereby avoiding stepping into cryptographic pitfalls), multiple encryption will definitely be more worth the effort as it will make things so much more
difficult to decrypt than any language-mixing you might attempt to do on your plaintext message before actually encrypting it.