On one hand, the language of worship doesn't matter, as the liturgy as a whole should transcend rational experience. Then again, our holy Fathers Cyril and Methodius dedicated themselves to communicating the Faith in a language understood by the people.
It has been an axiom of Orthodox practice that things should be done in the language of the people. This goes back at least to Saints Cyril and Methodius who were sent to evangelise the Slavs, and from whom we get the term 'Cyrillic' for the alphabet today used by Serbs, Russians, Ukrainians, Belarussians, and others. It was seen as necessary that, if people were going to be able to engage in the Faith in a real and unobstructed way, they would need to do so in their own language.
“Language in Orthodoxy, then, is both primary and secondary as a consideration. And while there is no clear answer as to what language you might experience across the various Orthodox jurisdictions, in AROCWE here in Britain and Ireland, it will be English (or one of our other national languages) first.”
You may wonder, then, what language gets used in Orthodox liturgies today, and the answer is: it depends.
There are Russian Orthodox communities in the English-speaking world that use Slavonic for services and Russian or Ukrainian over coffee. There are Greek Orthodox communities that use Greek for everything despite having been in a Western European country for more than a hundred years. But regardless, all Orthodox communities should be places of welcome to all people, and it is for this reason that the Archdiocese of Russian Orthodox Churches in Western Europe - although it gratefully receives and continues to live its Russian inheritance - seeks to worship and operate in the language of the land in which it finds itself. So, being Paris-based as it is, the first language of the Archdiocese is French. Outside of France, then, its parishes tend to use whatever the language of the country may be: in Germany, German; Italy, Italian; Great Britain and Ireland, English, Welsh, and Gaelic. But also, these same communities will be happy to use Russian, Ukrainian, Romanian, Greek, or any other language that seems pastorally appropriate according to the numbers that attend.
Language in Orthodoxy, then, is both primary and secondary as a consideration. And while there is no clear answer as to what language you might experience across the various Orthodox jurisdictions, in AROCWE here in Britain and Ireland, it will be English (or one of our other national languages) first.