Galko's skin got really dark as a result of tanning.
Otako comments that since Galko's skin has tanned so much, her nipples must be dark as well. Along with Ojou, Otako yells that Galko has dark nipples.
They say it's most common 
in people with big breasts.
يُقال أنّ الأمر يكون أكثر شيوعاً عند ذوات الأثداء الكبيرة
Soll bei Leuten mit großen Brüsten
am gängigsten sein.
La mayoría que sufre de 
eso tiene los pechos grandes.
Et en fait, c’est fréquent
chez les femmes à forte poitrine.
E senti qua: pare che sia piuttosto comune
 tra le ragazze con il seno grandi.
Dizem que é mais comum 
em gente com peitos grandes.