How to write if diacritic isnʼt avaible?
I know how add and use Czech keyboard, but I wonder: is here any standart with only English letters? Example of German: letter with umlaut → letter + e.
How would you write č, í etc?
The Czech language doesn't use any transliteration for letters with diacritics - if necessary, we just ignore the diacritics. Some people actually don't bother with typing the diacritics in chat and other informal texts. The resulting text is a bit harder to read, but mostly works.
I see cz for č all the time, not because the letter cannot be formed by the keyboard, but because some forum/board software will not show it correctly. For example, at a trivia site I participate in, we use cz because the letter č will not show up correctly in the various teams' message boards, even when one uses a Czech keyboard.
Not that we all speak Czech of course, but there is a player from Czechia on our team, and some of us are learning her language. And we write trivia questions about words in various languages, include Czech!
As has been mentioned, the alternative is simply to do without the diacritics.
Years ago I asked a Czech friend what would happen if you wrote without diacritics and he said: ”Nothing, people will just think you're abroad and don't have access to a Czech keyboard”.
In that sense it is sort of like Arabic or Hebrew, where the vowel marks are mostly superfluous for fluent speakers.
I do find it a lot more difficult to read without diacritics though, and there is a good chance that I will mispronounce any personal names or words that I am unfamiliar with.