There's something fascinating about how languages capture human vanity differently. The most direct translation would be 'self-conceit' or 'narcissism', but those feel too clinical. In casual conversations, I've heard 'big-headed' used playfully among friends - like when someone brags about their cooking skills but burns the toast.
Interestingly, British English tends toward 'up themselves' while Americans might say 'full of themselves'. Pop culture gives us great examples too - think Gaston from 'Beauty and the Beast' belting 'I'm especially good at expectorating!' That scene perfectly illustrates how self-absorption can be both ridiculous and entertaining.
翻訳サイトで調べると『自惚れる』は『self-conceited』や『vain』と出てきますが、実際の会話ではもっとニュアンス豊かな表現が使われますね。ネイティブがよく使うのは『full of oneself』——例えば『He's so full of himself after getting promoted』(昇進してから調子に乗ってる)みたいに、嫌味っぽさも含めて使えます。
『Narcissistic』も最近は流行語で、SNSで自撮りばかり投稿する人を『She's being narcissistic』なんて言ったり。ギリシャ神話のナルキッソス由来の単語で、臨床心理学用語から日常会話に広がった面白い例です。『Tokyo Ghoul』の壁井ユクが『自己愛の塊』と評される時、英語圏のファンは『walking narcissism』なんて表現してましたね。