Translation:The language of the Czech Republic is Czech.
The ĥ -sound [x] is the rarest in Esperanto.
"Ĥ was always the least used Esperanto letter/sound (though it usually has more dictionary entries than ĵ), and most of its uses are in Greek etyms, where it represented chi. Since the latter is pronounced [k] in most languages, neologistic equivalents soon appeared in which "ĥ" was replaced by "k", such as teĥniko → tekniko ("technology") and ĥemio → kemio ("chemistry"). Some other ĥ-replacements followed unusual patterns, such as ĥino → ĉino ("Chinese [person]").
These additions and replacements came very early and were in general use by World War I. Since then the imminent demise of ĥ has been often discussed, but has never really happened. There are very few modern ĥ-replacements, notably koruso for ĥoro ("chorus"). Some ĥ-words are preferred to existing replacements (old or new), such as ĥaoso vs. kaoso ("chaos").
Several words commonly use ĥ, particularly those of non-Greek etymology (ĥano ("khan"), ĥoto ("jota"), Liĥtenŝtejno ("Lichtenstein"), etc.) or those in which there is another word that uses "k" in that context. The latter include
eĥo ("echo") — eko ("beginning")
ĉeĥo ("Czech") — ĉeko ("bank check")
ĥoro ("chorus") — koro ("heart"), horo ("hour")"
Ĉeĥa is a noun that ends in "a" or is an adjective that can be used as noun?
It's an adjective - here, it's short for la ĉeĥa lingvo "the Czech language".
Languages are almost always referred to like this, i.e. just la + adjective, with lingvo omitted; as if people say "I can speak the English one and I'm learning the French one" (Mi scipovas paroli la anglan kaj mi lernas la francan). Esperanto is an exception as it's a proper noun.