>> In principle a cleaner implementation is possible in luatex (without using
>> active characters), but I am not completely sure about this. It will be
>> better to handle this as part of \mnum macro.
>>
> Hm, can you explain this a bit ?

Currently ConTeXt already checks if '.' is part of a complete number or 
not as part of the export macros. For example:

\m{x + 1.25y = 3.5}

exports as

   <m:math 
display="inline"><m:mrow><m:mi>𝑥</m:mi><m:mo>+</m:mo><m:mn>1.25</m:mn><m:mi>𝑦</m:mi><m:mo>=</m:mo><m:mn>3.5</m:mn></m:mrow></m:math>

Notice that 1.25 is part of a single <m:mn> tag.

In contrast, if you use comma as a math delimiter, then the export 
function does not recognize the number correctly.

\m{x + 1,25y = 3.5}

exports as

   <m:math 
display="inline"><m:mrow><m:mi>𝑥</m:mi><m:mo>+</m:mo><m:mn>1</m:mn><m:mo>,</m:mo><m:mn>25</m:mn><m:mi>𝑦</m:mi><m:mo>=</m:mo><m:mn>3</m:mn><m:mo>,</m:mo><m:mn>5</m:mn></m:mrow></m:math>

However, we do have a \mn macro (math number) to tag numbers. So you can 
type

\m{x + \mn{1,25}y = \mn{3,5}}

which exports as

   <m:math 
display="inline"><m:mrow><m:mi>𝑥</m:mi><m:mo>+</m:mo><m:mn>1,25</m:mn><m:mi>𝑦</m:mi><m:mo>=</m:mo><m:mn>3,5</m:mn></m:mrow></m:math>


So, if we want to recognize ',' as a decimal separator, two solutions are 
possible in MkIV. The first is to build upon the existing export code that 
recognizes '.' as a decimal separator (to figure out the contents of the 
<mn> tag) to also recognize ',' as a decimal separator;

The drawback of such an approach is that if you are in the habit of not 
adding a space after a comma, for example writing "Let $\naturalnumbers$ 
denote the set $\{1,2,3,\dots\}$ of natural numbers..." then you will get 
wrong output.

So, an alternative option is to ask the user to tag all numbers with \mn. 
Then, the \mn{...} macro can redefine the comma so that the spaces around 
it match its use as a decimal separator.

Aditya