Step 1: Analyzing option (A)
The idea that some languages are primitive and incapable of expressing complex thought is a myth. All natural human languages are equally expressive, even if they differ in vocabulary or script.
Step 2: Analyzing option (B)
The belief that slang, texting, or colloquial speech "degrades" a language is also a myth. Informal registers are a natural part of linguistic evolution and do not harm the structure of a language.
Step 3: Checking option (C)
This statement is actually true, not a myth. Languages exist independently of writing. Having or not having a script does not change their linguistic complexity, but saying "no difference" refers to equality in expressive power, which is correct.
Step 4: Checking option (D)
Grammar textbooks do prescribe rules and norms for "standard" usage. While linguists study language descriptively, prescriptive grammar books exist and do set such norms. Hence this is not a myth.
Step 5: Conclusion
The myths about language are only in (A) and (B).
\[
\boxed{Correct Answer: (A) and (B)}
\]