The problem with high-IQ characters picking up native fluency in a dozen languages with just a few points was addressed in GURPS Compendium I. The optional rule I use is that for purposes of learning languages, all characters have an IQ of 10, plus any levels of Language Talent. The exception is the character's native language, which is still learned from a Default of the character's IQ, plus Language Talent.
Bard: As per B47, if you are using your Bard skill in a language besides your native language, your effective Bard skill is reduced by 1 for every point below 12 in that language.
Poetry: Suffers the same penalties as Bard.
Additionally, your effective skill in Bard, Poetry and Writing can never exceed your skill in the language being used by more than 5. This includes your native language. So, if your skill in your native language is 10, your effective writing skill cannot exceed 15.
Defaulting from other languages: When a language is listed as defaulting from another language, this does not mean, for instance, that an English speaker who has never studied German can "guess" the correct German phrase for "Please get me to a doctor". It means that there is sufficient similarity between the languages (in the way of similar words, grammatical structures, and other parallels) that a speaker of one language may be able to get a concept across to a speaker of the other language. When relying on default rolls, a roll should be made for each single concept. The GM should determine what constitutes a "single concept"; generally it is no more than what can be contained in a simple sentence. "Where is the bathroom?" can be expressed with one language roll; "How do we find a back door into the computerized security system?" cannot.
[In the future, I intend to construct a list of language defaults. My old comparative linguistics textbooks ought to be good for something!]