Hi fellow DMs!

I am facing a language difficulty that I didn't realize would bother me that much.

I am French, mother language is French, but I live in Iceland. I have created and DMed a campaign for a few French-speaking DnDers living here (which is a way for me to not lose my language). But recently, a group of Icelandic friends asked my to DM that campaign for them.

But the problem is... my vocabulary in Icelandic is far from being as nuanced and expansive as in my mother language. I have made a first session with my Icelander group, and I can hear that I sound like a 5-year old, which bothers me quite a lot. To be sure that I was understood properly, I asked the players to describe how they imagined what I described. And the results were very disappointing, far from what I was trying to convey (and sometimes very different ideas in mind...). 

Has anyone been in that kind of situation before? Does it get better with time or am I just overthinking things?

I haven't been in your exact situation, but nevertheless, of course it gets better, the better you get at talking icelandic.

Now I don't know how good the icelandic folks are at english, but it seems to me that you are pretty fluent in english, so maybe use that for the more colorful decriptions, and then use icelandic for like normal conversations with NPC's and such. Or when you feel that you are missing the words in icelandic, sprinkel it with english.

That way you are training both languages :)

Just the fact that you are talking 3 languages is already pretty amazing and better than most people in the world. :)

I'm also french but DM in english as well and my mastery of the language improved so much since i started online gaming a couple years ago.  Don't hesitate to rely on written form to communicate when uncertain to get understood, or to pass critical infos as it will ensure you're all on the same page.

Bring visual aids...

My second language is English, but I have spoken it for a very long time, so I almost don't think of it as secondary anymore. 

But first off - Well done for taking on the challenge! There's no better way in improving language skills than being in a situation where you have to find a way to convey something - even if it means describing it in basic terms. 

I think the key question is whether you and your friends are having fun? If so, then I think you should push ahead, and enjoy the experience of finding a new motivation to improve your language!

If it is really, really getting in the way, where things are being misunderstood - then perhaps ask them if switching to English (or French) is an option for descriptions, while keeping the conversation about rules and events going in icelandic perhaps? That way, you are still keeping the most conversationalist part of the game in the native language, but the floral descriptions that tends to use quite concise words to build a picture is managed more straightforward. 

well the best way to Improving your Icelandic is to hear it spoken by a native speaker wouldn't you say?
And you have exactly that in your Icelandic group, I'm quite confident they are fully aware your not a native speaker and not particularly bothered by the fact.

What I would suggest is maybe adjust your DM'ing style a little by trying to get your players the describe scenes themselves, let them introduce the details and try and work those in ie
initially give the basic outline of the scene... its a bar with several patrons... now ask the players what they see ask them to describe the bar the patrons etc...

this way you'll gain insight into your players imagination and how it is expressed in Icelandic  

Edit -

Omitted an important aspect :o
Once players have given you some details of a scene repeat them, so your practising the pronunciation and internalising the new details