Most european languages generally agree on a common basic rules for the use of capital letters (A, B, C etc. as opposed to a, b, c...). While those may be joined by others, these basic rules exist in almost all (if not all) european languages.
It seems thus logical to apply the same rules in a common european language. These are:

The first letter in a sentence is a capital
Names of persons begin with a capital letter
Fix names, such as those of buildings, works of arts (including book titles), historical events etc. are written with capitalized initial letters.
Names of companies and organisations may begin with, consist of, or lack capitalized letters, the decision is usuallly done by the bearer of the name. The same is true for names used on the internet, artistic pseudonyms etc.

In the Latin names of species it may be a good idea to capitalize the initial letter of both genus and species name, differing from the official international guidelines for biological nomenclature.
However, this method would make biological names recognizable as foreign in a text written in the european language and the names are still readable. The official soultion to mark latin species names as foreign by wirting them in italics is problemativc due to the lack of this option in type machine and hand writing, to name the most important.