How to add a language visible from another language?
Maybe my "description" above is not clear, so I explain it detailed here: When I check a profile, I used to see that the user has a language there from another "language universe of Duolingo" (namely the Spanish-based, not the English-based where we are now). I checked more profiles where I saw that the Catalan is studied. Some has it visible there, some not. You can see that in my "signature", but not in my profile (under "Achievement"), unless I change the language of Duolingo to Spanish. I know that you have some scripts, I've never used them, but I am not so sure, whether that is because of the script. How could I put Catalan to my "English list"? And if that depends on a script, then on which?
You can't. You have to change source language to choose target languages from that language. So change your source language to Spanish if you want to learn Catalan. If you are going back and forth a lot, then you can try the user script:
This just speeds up the process - but you still end up changing your source language. This means that the UI of Duolingo will change.
The web site assumes that you have only one source language and displays your profile accordingly. Only languages from your current source language are shown.
In forums, the flags displayed are inconsistent. Sometimes it includes all your languages - other times not.
There is some comment somewhere that they are aware of the problem and will think about improving support for this sort of thing in the future. However, their developments appear to target making things easier and simpler for new learners, so I wouldn't hold your breath.
But then how is that, that I know some users here, who study Catalan on the Spanish side and when I check their Achievements on the English side, I do see their Catalan on the list, and there are some, where I don't see, that is the miracle for me.... How come?
When you say achievements, I assume you mean the list of languages on the profile page that shows the level and XP totals. How are you determining that they have switched to English? (I assume this is what you mean by 'the English side' but I am not sure). If I saw Catalan on a profile page I would assume that person had set their language to Spanish at the moment.
If you are seeing other languages listed, it may be that they are doing other courses from Spanish. In addition to Catalan, it would possible for them to have added the following target languages: French; German; Italian; Portuguese and of course English. Do you see anyone with more than these showing at the same time as Catalan?
Quite a few people target the same language from multiple source languages. So they may have completed French for English speakers, and have now done French for Spanish speakers. This allows them to practice Spanish and French at the same time. It also mean that French will show with Catalan, on their profile, if they start studying Catalan for Spanish speakers.
I just finished the Spanish tree, but Catalan is still unvisible. I found another user, who finished Spanish ages ago, having also Catalan, but that was also unvisible... So that is not a solution here :(
Replying to your other more recent comment...
I would not expect completing your Spanish tree to have an effect.
Catalan will only be visible on your profile, when you change the site language to Spanish. But then all the trees being learnt from English will disappear.