In the first part of this article, I pointed to EU's economically sub-optimal policy of language pluralism which results in costly overheads (see also  ) and reduces efficiency of multi-national working environments and reduces labour mobility across EU.
For example, one of less represented, (one may say subdued) EU studies of economics of language points to some of the economic benefits of English as a second language  in spite of EU's more public official policy on language diversity :
“If there is one language which Europe needs, that is plurilingualism.” (ibid),
and the related guidelines for migrants to adopt the local language of the country they move into, thus, though enabling their better local integration, but, simultaneously, restricting their further mobility (see ).
I there also introduced a rather rough measure of efficiency of written forms of languages (their orthography) as a communication channel, as a measurement of their relative parsimony in relation to other languages' translations representing same (or nearly same) message, and where I already pointed to some relative advantages of English over French.
Following-up this study in language efficiency, using simple google translator of a sample extract from my previous article on Sound Creativity, I found that the most of the official EU languages require longer texts, thus more characters to represent the same message. This is across the board valid for all Germanic and Romance groups' ones as well as for odd ones such as Basque, Greek, Albanian Bulgarian and, including Esperanto.
What came as a no surprise is that good part of South-west EU Slavic languages (Slovenian, Croat and Czech) required significantly less characters to convey the same message than English: Croat is another example of near-phonetically written language with noun cases declensions together with other languages of Serbo-Croat group where, for example Serbian translation, when written fully-phonetically using Cyrillic alphabet, yielded even smaller number of characters as it needed no dual-characters for any single sound.
This was not so unexpected because these languages use cases (noun cases declensions) which makes redundant (and thus do not require) many of the auxiliary prepositions (such as: to, from, by, of,....) that are used in most of EU languages that did-away or never had such noun cases declensions - thus, including all of the modern Romance but also most of Germanic languages. Polish, Bulgarian and Slovak however, despite noun cases declensions, yielded significantly more characters, this probably due to many of longer words they use.
Another reason is likely to be the XIXC reformation and simplification of, at the time Serbian (and later adopted in what became Serbo-Croat), in particular aimed at rationalising its writing so to transform it into a fully phonetically written language where, ideally, only one character represents one sound whilst, at the same time, utilising about 4 more characters in their alphabet than English.
However, in addition to language efficiency based on parsimony of its written form, some of many other economic dimensions and factors that need to be taken into account are the rigidity of the current system to change, the levels of currently already committed investment in IP (intellectual property) and education, costs of change and, related to it, current spread of the communication medium, i.e. the language.
If, for example we take into account that only 10 mil. Europeans speak Czech and about 5 mil. Croat (in total of about 15 mil. of all speakers of the all new variants of Serbo-Croat in Europe, though majority still outside EU), than that is all together less than 10% of EU citizens and it is therefore unlikely that one of their languages will be the EU “chosen” one despite their higher parsimony and processing efficiency.
There are probably at least two additional reasons: the dominant non-English speaking groups of EU countries are speaking Romance and Germanic languages which are rather different from Slavic, and thus, it is more difficult for their speakers to learn a Slavic one, and also, many of those countries, especially those relatively smaller ones, have already invested a great deal in learning English as their second language (e.g. Scandinavian ones).
However, the fault lines between Western Roman Empire, Eastern Roman Empire on the South-East and what were lands of so called barbarians on the North-East of Europe, still render its three main cultural and linguistic clusters of influence and prospect of UK exit from EU will inevitably render some difficulties for future of English as one of the official EU working languages, or for its further spread within EU.
In any case, as mentioned earlier, this is just a preliminary, rough and simplified analysis of written forms of EU languages which aims mainly to initiate a further, more precise and comprehensive analysis and discussion. But, before the European fault lines render its further division and we burn the Treaty of Rome to build another, (only) South-West European Roman Empire from its ashes, there may be other solutions on the plate. But, even if EU start seeking further integration, it is probably unlikely that we will see a wider political support for a single administrative language, even as compulsory second one, until EU parliament politicians become required to campaign and seek support from voters throughout EU rather than just within their national, language defined, boundaries.
 BBC: Fast-talking MEPs urged to slow down for interpreters, http://www.bbc.com/news/world-europe-35501198
 Grin, F: Using Language Economics and Education Economics in Language Education Policy, Council of Europe Language Policy Unit , 2002.
 Languages for democracy and social cohesion Diversity, equity and quality, https://www.coe.int/t/dg4/linguistic/Source/Source2014/LPU-60ans_EN.pdf
 Beacco et al. : LINGUISTIC INTEGRATION OF ADULT MIGRANTS Guide to policy development and implementation, Council of Europe Language Policy Unit , 2014