Translating CMS to Swiss German

Hi there. I am currently developing a multilingual site - among others in Swiss German. The site is aimed at Swiss-German site visitors in Switzerland, so the language icon matters. I wanted to translate the site interface (we have implemented some custom packages), but I do not have a Swiss German language string to translate. It says “n.v.”
I have made a copy of German German language mo- and po-file , renamded it to de_CH and uploaded it to application/languages/site ---- but nothing shows.
How can I get this done?

this post might help:

I think that some time ago user @mlocati had a post in the old forums or in github about adding swiss language .po files. But I can’t find it.