Why all Maths should be taught through 

the Medium of English in Wales - hgunn.uk

  Much of the mathematics that is learnt in school will not be presented in neatly presented exercise books in children's future everyday lives.  Learning to apply mathematics amounts to much more than doing sums, calculations.

The mathematical problems that children need to apply will need to be extracted from everyday language.  Even the simple four number operations like  'add', 'subtract' , 'multiply' and 'divide' do not arise in everyday language.  ('Divide' may used a times). Children need to detect from everyday language what mathematical operations to apply.
  Learning is a process of cognitive growth.  There learning  of multiplication tables is similar to learning a vocabulary of words in a new language. They can be worked out, but it requires a considerable amount of practice before children will be able to fluently recall them, through recalling them in their tables or through applied mathematics.
         In practice what happens, as with language words, is that the easier tables that are regularly  applied will be the 
ones learnt from an early age, such as 3 x 3 = 9 will tend to be fluently retained. When we need to add three set of three, we will automatically bring the total of nine into our heads. Even government education ministers have had 'red faces' when they have been asked to work a multiples out and got it wrong!
  Mathematics also relates to creating a way of thinking precisely. Probability, for instance, can be calculated, but when it is apply in everyday life it will normally  only need to be estimated, such when we work orut the probability of something happening. This should be applied automatically. Many people do not do so.
  There is the question that cognitive psychologist pose, where they ask, "
How many animals did Moses place into his ark".  Most of the time they will receive an
numeric 
answer, despite the fact it was 'Noah' who had an ark. This illustrated how automatic well learnt information is grown in our memories , which we able to call instantly without needing to give detailed thought to it. This is the ideal of all learning.
         A monolingual Welsh speaker would be unable to make that mistake when it is presented it in English. The situation would be more blurred with bilingual speakers. It is unlikely that they encounter the concept in each context at the same level of frequency.  Neural links will need to be grown in each language for it to be automatically processed in each language. 
   The risk of expecting children to learn mathematics in a minority language is that there will be very limited opportunities to use the mathematics they learn in it. Mathematics also makes demands upon working memory. Lack of fluency in a unfamiliar language will make learning maths more demanding, especially for children of lower working memory capacity.

Plenary

    The most convincing reason why children should learn maths through the medium of English is because there will be few opportunities for children to apply mathematics in the Welsh language and take Welsh language higher education courses that would require mathematics to be applied in Welsh.