is that difference only with the letter
"e"?
If yes, why can't it be decided that some people pronounce it
as "je" (like they do in Russian, they just write "е" and not
"йе")
and some other people pronounce it as "e", and keep a single way of writting?
There are some more differences...
In 12th century so called "new dialects of Serbian/Croatian language"
are devided over different reflexes of old vocal "jat" (capital letter
"Ѣ", small letter "ѣ" in Cyrillic, or Ě / ě in Latin) into: ekavian,
iyekavian and ikavian variant. Ekavian reflex is almost always "e"
(sometimes it is "i"), ikavian reflex is always "i", and iyekavian
reflexes are in general "ye" or "iye", but often "e" and
rare "i".
After 12th century there some consonant changes was happened. When
iyekavian reflex "ye" stayed after consonants 'l' or 'n', string
"lje"
(lye) or "nje" (nye) became two, not three volwes and "lj" and
"nj"
was not anymore "l+j" or "n+j", but "lj" and "nj"
as soft "l" and "n".
So, differences became bigger, but not so big. Ekavians and iyekavians
are closer between themselfs then with any other language speakers.
For example, when iyekavian Croat from Zagreb or iyekavian Serb from
Banja Luka talks with ekavian Serb from Belgrade, they will not have
any problem in communication.
However, in writting systems there are some differences. Not only
"mleko-mlijeko", but, for example, word for "notepad" in ekavian
variant is "beležnica", but in iyekavian it is "bilježnica". Old form
was "bělěžnica" (before "jat" transformation, after other changes in
language). Ekavians changed "ě" into e, but iyekavians changed first
"ě" into 'i' and second into "ye". As /j/ made 'l'
soft, "lj"
combination dosn't represent two volwes, but one. Around 20.000
lexemes (and around 200.000 forms/words) are affected with "jat"
transformation.
Vuk Karadzic and Ljudevit Gaj sancionied only ekavian and iyekavian,
but not ikavian. So, today Croatian and Bosnian have only iyekavian
varian (a lot of Croats and some of Bosniaks are ikavian) and Serbian
has iyekavian and ekavian.
or the other way around, always write the
"j", but some people not
pronounce it.
having two different writting systems just for that (which means four
different writting systemns in total) seems unjustified;
There are a lot of possibilities for solving that problem. However,
you and me are not the persons who are making those decisions. Also, I
think that a lot of Serbian linguists don't think a lot about
complicated situation with standards. A lot of them would use Cyrillic
or Latin (or both if they don't care about alphabet) and one dialect.
Ortography of Serbian Language is written only in Cyrillic in two
variants (ekavian and iyekavian)...
The other question is that Serbian (Croatian and Bosnian) language
have (with Finish and, I think, Estonian) the most phonetic alphabets.
This is two centuries tradition of Serbian language and I am the first
who would be against morphological writing system (as well as other
linguists). Morphological solution is introduction of "jat" into
ortography (Ěě in Latin, Ѣѣ in Cyrillic), but a lot of Ekavians would
not know where to put it (not all 'e' came from 'jat'); actualy, only
Ekavians with high linguistic education know where to put 'jat' and
where not to put.
About other way, I said that Karadzic's nazi regime tried to do that.
I can say that cultures are complicated. They are not based on bits
and formal logic. Hindi and Urdu are the same linguistic language, as
well and Serbian, Croatian and Bosnian. However, those are different
cultures; and all of that cultures has set of their own rules. 10
million Serbs has four writting systems. But, 500.000 of Lusatian
Serbs has two writting systems :) Or, almost 5 million inhabitans of
Papua New Guinea has around 800 different (!) languages!