Obviously no, 26² makes 676, and number of languages in the world is estimated to be around 4000 and 8000. Given that 26³ is 17576, you would probably start to have something vast enough with scheme like ab-cd-ef.

The example given is de-CH and lb-LU - a counterexample for the neat scheme would require separate dialects of the same language within a single country e.g. 近畿方言.