Let's be honest: it looks like shit and interrupts the flow of a sentence. The alternative of writing both words completely also makes sentences way longer than they should be.
Every gendered language would have to make massive changes to become ungendered and change their grammar too. There's quite a large list of ungendered languages.
German, to my knowledge, is like Russian and has cases which change the ending of a noun depending on the purpose in the sentence (subject, direct object, indirect object, possessor, location, time, ...). Languages with only male and female would have to add a neutral ending, and languages with 3 grammatical genders would have to either use the neutral ending - if there is one, or make a new one specific to living beings.
Then of course pronouns would have to be changed too. In English they/them is already confusing enough when talking about a singular person to somebody and the person doesn't know it's a single person e.g "I talked to them today" - a group or a person? Until hints are dropped it isn't clear. The most logical would've been "it", but that's used for inanimate objects. I'm sure there's a neutral third person singular pronoun languages could borrow instead of using the second person plural.