Suppose a person speaks several languages, occasionally making mistakes in grammar(s), using untypical patterns, clichés and/or calques.
Can such a level of language competence be defined as bilingualism/multilingualism?
Do bilingual speakers ever make language slips?
Do multilingual persons make language mistakes more often than bilingual or monolingual speakers?
Hope all can agree that mistakes can be made at any level of proficiency. The level(s) of competence needed to make a reasonable claim for bi-/multilingualism are of course flexible but can to some degree be measured. One example is the CEFR (Common European Framework of Reference for Languages) but there are other applicable standards.
As to the error rates based on the number of languages spoken, I guess you're referring to the speaker's best language(s). Errors made in a second(+) language would seem to only be a crude measure of how well that language is known. I haven't seen this precise question studied but it's somewhat similar to the question of whether bilingual children develop language skills later than their monolingual counterparts. It seems to have been the conventional wisdom that there is a delay but studies have not shown this conclusively. This of course doesn't mean that errors rates in primary language are also comparable but I would doubt that the effect is large.
I can only answer your first two questions, but: Native or very high-level speakers of languages make mistakes all the time. I don't see why making the occasional mistake in several languages would prevent someone from being defined as multilingual. And of course bilinguals slip up sometimes. Everyone does!
This is interesting question.
I'm sorry to post here, but I'd like to share my experience.
I do speak three languages fluently, and those three are from different language categories; Japanese, Indonesian, and English. I also understand Hawaiian, but not as fluent as other three.
In my opinion, grammatical mistakes are more based on the educational background on the language, but are not caused by multilingualism. Most monolingual people make more grammartical mistakes than I do, in my experience.
Actually, English is my third language, but because I have had the most intensive training on English grammars, I'm confident when I write/read/speak in English more than other two.
And, most people never care about how correct the grammar is, when they are just chatting.
