Owl - I hope you don't mind me picking your brain a bit because I won't have a lot of time to fire up Arma myself and dig into this issue this week. First questions that come to my mind:
1) Are Czech voices working in the original CSLA characters?
2) Are Czech voices working in the CSLA characters that I haven't modded, i.e. where I've used a CSLA classname to crew a SOG static weapon?
3) Is the lack of Czech voices limited to the new character classnames I've created, i.e. US tank crew and riflemen crewing the desert retextured vehicles? Edit: lol, I suppose that was a trick question :D Now that I think about it, I haven't created any classnames that should speak Czech.

1) No
2) No
3) No, even original units like the rifleman and all that don't speak Czech

They default to American English, both in Editor and Zeus

Could you show me the mods you've got loaded when this happens? If it's a long list maybe just post links to however many screenshots it takes to page through them all. Feel free to track me down in any of the CDLC discords, send me a DM and then paste the screenshots in there.

Yeah well I have 161 mods in use so if its one of them causing it It would be impossible to tell

I see there's a related issue reported in the Czech Voices mod's comments section... https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/comments/2545433942

Doing some testing myself though, and I did encounter the issue, just not as widespread as you're seeing. For instance, in a simple mission on the VR map containing just a single group of characters - one of each from the CSLA faction's "Men (Crewmen and Pilots)" editor subcategory - once in-game and issuing orders, all voices were speaking Czech except for the tank commander and the tank officer (classnames CSLA_tnkCmd & CSLA_tnkOfc). That's as far as I tested with my mod loaded. For the record: I was also running all 4 CDLCs and all the mod recommendations listed on my front page.

Next, I unloaded all mods except Czech Voices, but kept all 4 CDLCs. Then built the same mission again. The same two classnames spoke English; the rest Czech.

And finally, one last quick test, same as the last, but with only the CSLA CDLC. Exact same result.

So it seems like there are some classnames that the Czech voices mod misses for some reason, with or without my mod. I never did encounter your issue, of having all CSLA characters revert to English, so I assume some other mod is causing that for you.

I'll report my findings to the Czech Voices author.