Why should speakers use their mother tongue even if they know other languages?

I once read somewhere that in general, people appear 30% less intelligent when they speak a foreign language as compared to speaking their mother tongue. Can't remember the source but that that is a very convincing argument!

I went to see the movie on Hannah Arendt, the German philosopher, yesterday. She was portrayed speaking English with a very heavy German accent. One of her friends comments on this in the movie and tells another listener "Hannah speaking English is like she is playing the fiddle - if you want to hear her play the Stradivarius, you should learn German" (or words to that effect). Maybe we can use this comparison trying to explain to conference organisers and speakers why it is much better for them to speak their mother tongue.

We are discussing here whether a foreign speaker must use his native tongue or, supposing he knows the language of the audience, speak in that language.

To answer this question I need to refer to some theory concerning the notion of "Coordinate Bilinguals" and "Compound or Balanced Bilinguals". The latter are those who have age-appropriate competency in both languages, with the ensuing cognitive benefits: same level of oral and written creativity fluency and flexibility in both languages, heightened cultural knowledge, accurate delivery and pronunciation; and, above all, that can feel in both languages.

So, unless the speaker is a Balanced Bilingual his command of the second language will not be as good as that of his/her native tongue which will inevitable reduce his performance rate, making the message more difficult to convey to the audience in the case of interpreters, and more difficult to grasp, in the case of the audience.

...in a nutshell and as someone once said, in one's mother tongue one says what one wants, in other languages but what one can! ...You may be able to say a lot, but you'll always be limited by what you can say...and there's no way of knowing beforehand exactly what level of language will be required, once one engages in situational speech. Would anyone willingly put on leg irons before starting to run?

This article from The Wall Street Journal (kindly shared by Katty Kauffman) helps understand how Americans feel about non-native speakers of English:

Is proper English dying? And should us care?

And this interview from L'Express (kindly shared by Etienne van Dam) elaborates on the link between language and thought:

Imposer sa langue, c'est imposer sa pensée

But if your mother tongue is hindi or arabic... what would happen? In South America we have no local interpreters for those languages (and Russian, Chinese and so on...) so foreign visitors must speak in an acquired language. This is where we can help our speakers because some are scared stiff of the experience, others do not want to stray away from the script that they have prepared with a coach or advisor... as long as we are provided with an opportunity to meet with the speaker before the meeting starts!!

I recall a scene from The Interpreters: A Historical Perspective - a documentary about interpreters at the UN. The then-Spanish ambassador to the UN says something tantamount to (and I'm working from memory here): if people must speak English (for example), then you send an EN speaker, not necessarily someone who best knows the subject. And you lose the rich cultural heritage that underly your language.

Communication is not just about language, but also culture. Why should we choose to forego all the wealth of cultural knowledge embedded in a language, especially if a choice is available (I'm assuming that the question implies a choice, i.e. that the speaker's native language is being interpreted at the meeting)?