Il a quand même une différence entre "Je me sens seul" et "Je suis seul" dans la majorité des langues au moins, comme le français ou l'anglais. Ici il y a les deux, s'est confondu, c'est dommage.

Hay una diferencia entre "me siento solo" y "estoy solo" en la majoria de los idiomas. Aqui tenemos los dos, entremazclados!

It could be great to part this in two different sentences: "I am alone" and "I feel alone".

You are talking about the difference between "Je me sens seul" (I feel the solitude) and "Je suis seul" (I'm not with anyone else), right? As far as I understand, this difference is expressed in English and German by different adjectives: "lonely" and "alone", "einsam" and "allein", unlike in French where the verbs make the difference. Whether the verb is be/sein or feel/fühlen, the sentence expresses the feeling of solitude (not the state of "être seul") if the adjective is "lonely"/"einsam". So I think these are right translations of the Japanese sentence, which also means the feeling of solitude, and there's no confusion here. (A Japanese equivalent for "Je suis seul" would be "私は一人です".)

danke für die Erklärung hi1811 !