Hi @Tony763 and welcome to this great place of fun and Mycrofting 
Czech isnt yet supported by mycroft-core. But there are work going on to get more languages supported.
You are correct that this could be better mentioned in the documentation. But under Languages in chapper 6. Mycroft Core - Lingua Franca library there is mentioned:
As of mid-2019, the languages supported by mycroft-core are:
- German (
de) - French (
fr) - Hungarian (
hu) - Italian (
it) - Portuguese (
pt) - Swedish (
sv)
There is also active progress being made on:
- Danish (
da) - Dutch (
nl) - Spanish (
es)
Contributions to this utility are warmly welcomed.
For skills main source of translations is comming from translate.mycroft.ai, so when a skill is translated the translations are sendt to the skil auther to merge into his skill.
That said, I am working on a auto translate skill that could help if/when a skill is missing some translations. And @gras64 is working on a skill that pulls down the translations from translation.mycroft.ai to get it faster than waiting on the skill auther to merge or if a translation is not 100% one could use what is done and fallback to english or auto translation.
But first mycroft-core has to be supporting the language.
So if yu are willing to help translation the main work has to be done in Lingua Franca Github repository where parsers formating for each language is made.