|For that matter, isn't Latin the same way?
|I wouldn't be surprised if Italian and
|Spanish and others got it from Latin.

Mother tongue[spanish] =>   'Lengua Materna'

Mother tongue[latin]       => "Lingua Mater"

Language[spanish] => Lenguaje
Language[latin]=> Linguarum

All of this are valid  expressions



|  Of course, in German and English, we have
|  the concept ("language"==="speaking organ")
|  though not the same root word as Italian
|  and others.

Same for latin derived languages....


IDIOM=any expression that consists of at least two words, which  is
expressed differently in
other language.

So it seems Mother tongue  is not an idiom ...

In spanish we say ("Yo tengo frio") what is exactly translated as "I
have cold"
In english ("to pull his leg")what is idiomatic translated into spanish
as "to take his hair"

What is interesting to me is how  'idiomatic' meanings are created
depending onthe 'value' of each of the words that it contains.

   'Mother tongue' is many-languages  valid expression because
  Mother and tongue  have  the same.... poetic (the best word I found)
meaning in all languages (built-in words )
  In contrast with  to "to pull his leg"  and "to take his hair" where
the poetic meaning of hair and leg  differs
  from the English culture and the Spanish culture due Spanish people
tends to be bald and English people tends to
  be Taller so they lost easier the equilibrium..

  ....I did not smoked anything wired mom !!

IMO  Same happens with programming lenguages....a lot to think
about..(regexs, flow control)



  The best way to know a population is knowing its idioms.
  JP Sartre.