One factor in the platonic vs romantic interpretations is the use of "suki", which we all know means "like''. "Suki" is an adjective and can be used in both a platonic and romantic sense. Generally, "suki" is used in the romantic way when referring to people.

Langa's mom seems to assume he's talking about having a crush on a girl who he got into a fight with, so she meant it as romantic. But let's give them the benefit of the doubt and assume that Langa interpreted his mom's use of "suki" as platonic. 

If Langa thought she meant "do you like this person?" in a friendly way (which he obviously does like Reki since they are close friends) his reaction feels odd. 

Normally, in these situations, this is where Langa would have replied "yeah, he's my best friend." Or even just "friend." But like we all saw, he's taken aback and surprised, hesitates and takes a moment to consider her question before replying "well... yeah." Like he's just realizing how he feels. Either this boy has the emotional self-awareness of a timbit and somehow is unable to say that he likes his best friend (in the platonic sense), or he is just realizing he has a crush on his best friend. 

He's shocked and surprised and hesitates to answer, thinking about it and turning away. Reki and Langa, before their "break up", were very close friends. Best friends. They have their own handshake, they made promises to eachother. They would go to school together, eat lunch together, go home together, go to work together, skate and practice together, go to "S'' together. They also went to the island with everyone else, but spent a good amount of time, just them, together. In the platonic sense, they DEFINITELY like each other. So, why would Langa hesitate to admit that he likes Reki as a friend?

There's also the whole "her" thing. When Langas mom says "You have to act on [your feelings] if you really care about her!" Langa's reaction is pretty interesting. When his mom said "if you really like her!" it told us, the depressed fuckers watching the show, that she meant her previous "do you like this person" question in a romantic sense. If she meant platonic, she would have kept her advice gender neutral and said "if you really care about them", or even assumed it was about Reki and used "him". She didn't. The only interpretation of that line is romantic.

With the previous parts of this scene I explained, the end can be interpreted as platonic, but it makes so much more sense romantically. 

Langa replying, dead straight face and confused ""her"?" can be taken a few ways. One is that he, somehow, STILL doesn't understand she meant it in a romantic sense. Another is that he is confused as to why she thinks it's a girl he's talking about. He doesn't say "no mom, it's my friend Reki, not a girl." He doesn't get all flustered when he realizes she meant as a crush. He's unphased by the crush implication and only questions the pronouns his mother used. They had their little back and forth "huh?" and then the scene changed. No clarification. 

Either this is some next-level queerbaiting or this is the 2021 equivalent of Yuri On Ice with a different type of skating.