Summary: No matter what language you’re writing in, you probably will find that you need to use words from another language, especially if your story takes place in the “Other” world. It’s not because you have to have a new language, but there are words in our world’s languages that don’t translate between languages, never mind between worlds.
There are a lot of pitfalls in writing science fiction stories, stories that take place in “The Other World”. Language, dialects, and culture are the first things that need to be sorted out because those are the first things people are going to notice about your characters.
In “Der Reizen”, it was easier for me to create the language as I needed it. I have a list, of course, of things that can happen and things that are ‘common’ in Tielen, but I let the characters tell me what was ‘culturally acceptable’ and what wasn’t. There were a few things that I needed to learn about dialects, too, both in Tiel and in German. I decided that both languages would be the “Hoch Deutsch” or the ‘proper English’ of the respective countries. It made research a bit easier and it made writing about the science fiction aspect much simpler. I could create words that were accurate descriptions of what I wanted but that didn’t take up a whole lot of printing room. Not only did I translate the unfamiliar words into English as I was writing the story, but I also added a glossary for those who prefer one.
Bavaria has a language all its own, but it’s a dialect of German. The only problem with creating a dialect is that you have to create the ‘proper’ language, too, especially if your characters need to communicate between regions of dialect. A number of SF writers have gotten around the problem of creating a new language by creating ‘Common’ or ‘Basic’ as a language. I don’t think I’ve ever heard someone ‘speak’ in that language, come to think of it, but it is one way of getting around the language barrier.
Now that you’ve created your language, how on earth are you going to use it in ‘normal speech’? What I did in “Der Reizen” was to use only the important words I needed, such as Diu Seeleocran for ‘the Seekers’. The language of the Tiel is a cross between Celtic, Latin and the German of the closest Gate between the worlds. If I needed to use either German or Tiel as a ‘primary language’ for my character’s speech then I use the grammar of the appropriate language rather than the actual words. It gets across the point of the difference in the language without going into a lot of translation. Having said that, though, there are some expressions in any language that a person will resort to when surprised or emphatic about something. Those expressions, those “Graiß God” words and phrases that you automatically use, can be used as-is. Depending on the effect you want, no translation is necessary. It’s something that everyone does when speaking another language, I think.
Not everyone is familiar with German or Tiel, but there are very few “English as a first language” speakers who don’t have a fairly good grasp of spoken English grammar. Something as simple as making literal translations of the words you need (for example, a ‘box of ice’ for an ‘icebox’) will create the impression of a second language without making the reader reach for a translation program or dictionary.