It's worth noting that semi-lingualism is a debunked pseudoscientific theory.
https://onlinelibrary.wiley.com/doi/10.1111/josl.12614
https://www.academia.edu/48879106/The_Myth_of_Semilingualism
The concept seems to be more prominent in Japan - see the second article - perhaps due to societal expectations about conformity and fitting in.
I think we need to clarify a few things. "Semilingualism" is a supposed condition where someone who is properly bilingual is perceived to be unable to properly speak the language that they are socially expected to be able to, while at the same time not being able to speak with complete fluency the second language they know because it isn't their mother tongue. The real cause for this is because a child who isn't raised from birth to be bilingual, if then dropped into a place with a different language from their mother tongue and loses contact with speakers of their mother tongue, will lose out on the vocabulary and grammatical developments a speaker of their mother tongue would be exposed to growing up. They'll be stuck with the linguistic capabilities of a child. At the same time they won't have full proficiency in their second language because they only started picking it up after entering the new place. A milder example is experienced by all international students. When they get back they'll have difficulty expressing the technical language of their specialization in their original language because they never learned those terms in that original language. It took me two years after coming back home to realize that "unduh" is Indonesian for "download". This condition is only considered a disorder in places where the social expectation is for anyone of the same nationality to be able to speak the national language like a proper adult, like in Japan. In the US where barely anyone speaks "proper English" anyway, people don't even realize such a problem exists. So "semilingualism" is just bilingualism that doesn't meet social expectations.
But this is not what Alice has. Her problem is the literal opposite.
What Alice has is a real developmental deficiency that does occur with bilingual or multilingual children whose linguistic development gets haywired in childhood. It's also sometimes tied to autism. Unlike the "semilingual" phenomenon which emerges out of social expectation, people who have this deficiency is unable to speak properly in any language EVER. What actually happens in this case is that a bilingual child is stuck with linguistic confusion because they're raised from birth in a bilingual environment but never gets to a point where they are taught to properly separate the two languages. Instead they think in a hybrid of both languages, so when they try to speak out one of the languages, there will be gaps.
The manga (mis)uses the term "semilingual" to name Alice's problem, which is inaccurate, but her condition is real. That's why I choose not to use the same term to describe her condition.