Hi, just my thought and idea on all of this ...
By creating Asgardia we have the chance to go in a different and unique way, unlike anything that has been done. Learn from humanity's history and extract only the best by clearly seeing and understanding the barriers and mistakes done before by our ancestors. Having said this, the language has always been the biggest barrier in the world. It has caused many wars and possibly many more to come. I believe that the world has plenty of languages already created and creating a whole new language from scratch as some fellow citizens proposed above would most definitely put just another barrier and another difficulty for our new society.
My personal opinion on this is that, as the situation currently stands, we should try and have as little barriers as possible both from a social point of view and from a technological point of view. For this reason we need a language which is already widely known by people, is easy to learn or is close to the majority of the languages which our citizens speak. Asgardia has to be an open society with an open mind, something that no nation has yet achieved yet.
Considering the above, our citizens speak 3 main groups of languages - Slavic (Russian, Polish, Eastern European in general), Latin (Spanish, English, German etc) and Mandarin (China).
Asgardia will be a nation of technology, development and the hub for human engineering and brilliant mind. We need to take this in account when considering which language to choose as official.
Bulgarian is my native language so this falls under the Slavic group, I also know Russian. The benefits of those language are that you have quite a lot of mechanical theory written in slavic and a big portion of today's electric engineering comes from Russia. However the majority of this literature is getting quite outdated and replaced by newer technologies. The big downside of this group is that it is harder to learn and pronounce to non-native speakers. This is one of the main reasons why Russian as a language did not do so well outside of Eastern Europe and didn't establish itself as a preferred language in countries like Thailand etc.
Mandarin is a great language however it is extremely extremely hard to learn and the logical implication of it technologically would be much harder as it has a lot of symbols and the syntax is so wide and hard to get your head around it.
Lastly, English, part of the Latin group. I think this is the most suitable language to have it as an official. It is the easiest to learn and is closer to a big variety of nationalities and most importantly it has huge technological implication. All programming languages work in English. The English syntax is used by every computer and every new technology. This will guarantee that our society will always be open to innovation and will speak in a way that everyone will support us and understand us. We will further be able to implement new tech much easier faster and more efficient. The benefits are many for me to list here.
As a conclusion, there are many linguistic studies which are oriented towards the future of the languages of the Earth and pretty much all of them estimate that with the current speed of globalization, In as little as 200-500 years, the majority of the languages from a same group would be almost the same and within 1000 years the chances are that people will speak one language only. Just think about how many words you have in your language, no matter what it is, which are the same as in English and now consider that the majority of those words have came about mainly in the past 50 years (or less), way before the creation of the internet and social media.
Choosing English only makes sense as this will be a symbol of unity in the world and choosing the efficient way instead of the traditional prideful way. I think we have to be different and make a difference by the way we think, act and operate, rather than the language we speak. This is just my opinion, there are many more things which I have missed and would like to add but I don't want to bore you too much :D