Glad is used with an omitted object. Which is to say that you are glad ABOUT something, even when this something is not stated. For example: "he has come to visit, hasn't he?" "yes, I'm glad" (that he has come to visit). Without an inferred object, the use of glad is not good English. So, no, "he is a glad person" is not correct.
"They" is commonly used as singular in English. It's an accepted usage by multiple dictionaries and useful when you don't know a person's gender. https://www.merriam-webster.com/dictionary/they
Since the turkish "O" is not gendered in any way, I believe "they" should be a valid translation! Especially because in this sentence, there's no way to know if "he" or "she" would be more appropriate.
I see your point. And I went through the link you suggested.
I still don't agree with your proposition to use 'they' when translating the Turkish 'O'.
First, if 'they' can be used when translating 'O', then you will be equating it with 'Onlar' which will then be redundant and of no use.
Second, if we assume that what you are proposing is accepted in English (and I believe it's not), then it doesn't necessarily mean that it should/would be accepted in another language (Turkish or otherwise). For example, as an accredited English-Arabic translator myself, I know that this is not accepted in Arabic. When the gender is not known, the generic (masculine) form is used rather than 'they'.
Third, there is a fundamental difference between composing text and translating text. When you are the author of a written or spoken text, you are free to communicate meaning in the way you sit fit. For this reason, yes, some people may resort to using 'they', in order to avoid the clumsiness of 'he/she' and 'his/her' throughout the text.
For example, instead of saying "If a student misses a class, he/she has to fill out a special form and contact his/her teacher", you can choose to say "If students miss a class, they have to fill out a special form and contact their teachers". It is your choice to do so, as the link you forwarded mentions. But this is not a translation exercise!
When you translate text, on the other hand, you are not the owner/author of the original text. You have no right to change the meaning of the original text. If the original text says one person, you cannot say two or more persons in the translation. This is a distortion of the original meaning in the original sentence. 'They' is not singular and is not accepted as a correct translation for a singular pronoun. In translation you need to stick to the original text.
In the real world (outside Duolingo) you can be taken to court for such an error! Imagine you translate written text or interpret a phone call relating to a crime and you change the singular pronouns in the original text you read/hear to plural pronouns! You would then be accused of misleading justice! This perhaps is an extreme example, but it shows how delicate translation is...