There is a mess in the names of spaces in Czech UI. The names of spaces are not consistent:

Em Space = "Em mezera" or "Dlouhá mezera"
En Space = "En mezera" or "Krátká mezera"
Nonbreaking Space = "Pevná mezera" or "Nedělitelná mezera"

Depending on the situation, the user sees either one or the other name for an identical space.

I suggest to:
1. Use "En mezera" and "Em mezera" only. It is not the traditional Czech name, it is an Anglicism, but quite widespread and understandable
2. Use "Čtverčíková mezera" and "Půlčtverčíková mezera", the traditional typographic terms, nevertheless some may find it obsolete

Terms "Dlouhá mezera" and "Krátká mezera" are confusing. It means "Long space" and "Short space". But how long? What does short means? It indicates that it is shorter than the default space, but it is actually wider. Both terms were never used in Czech language. Professionals use "Čtverčíková mezera", young people use "Em mezera". No one would ever use "Dlouhá mezera".

As for the nonbreaking spaces, correct options for "Nonbreaking space" and "Nonbreaking space (fixed width)" are:

Nedělitená mezera
Nedělitená mezera (pevná šířka)
or
Nezlomitelná mezera
Pevná mezera
or
Nedělitená mezera
Pevná mezera

Explanation:

"Nedělitelná mezera" or "Nezlomitelná mezera" is the correct term for Nonbreaking space of variable width

"Pevná mezera" is the correct name for Nonbreaking space of fixed width

"Nedělitelná mezera (fixed with)" is a weird neologism. It is something like "Horse (with stripes)" instead of "Zebra".

See also https://cs.wikipedia.org/wiki/Nezlomiteln%C3%A1_mezera