日本語と同様に、英語でも"half joking"で「冗談半分」です。
ただし、英語圏の人がこの表現を使う時の"half joking" 「冗談半分」は、"half serious"「本気半分」より本気に近いようです。直接言いにくい話をするよりもっと深刻さが伝わるような気がします。
例文）
"She was only half joking about leaving the company."
「彼女が会社を退職するって言ってたのは冗談半分だったよ。」
Semi serious = Definition of semiserious. : of a light nature but having a possible serious implication or interpretation : partly serious.
Jokingly = in a humorous or flippant manner.
"I commented jokingly that they could still win if they kicked a goal every minute"
Semi serious の意味は、軽い性質なのだが真面目な性格があるとき、半真面目
Jokingly = ユーモアな、軽々しい
例："I commented jokingly that they could still win if they kicked a goal every minute"
When you want to communicate to someone that you were only half joking, then you can say:
-I was half-joking/kidding
-I was not being serious
-I said that half-jokingly.
半分冗談だと言うことを人に言いたいのであれば、こう言うことが出来ます。
例文
-I was half-joking/kidding
半分冗談だった
-I was not being serious
本気ではなかった
-I said that half-jokingly.
半分冗談で言ったんだよ
I am saying it with jest, but it's not funny really
examples
"I was reading about the war, I had to make
a joke out of it because it was such a sad topic".
or
"many a truth is spoken in jest" a phrase
meaning to point out the truth using a joke.
or
" I am saying it in jest, but it's not funny really".
【例】
"I was reading about the war, I had to make
a joke out of it because it was such a sad topic".
(戦争の記事を読んでいて。ものすごく悲しい話だから、冗談にするしかなかった)
"many a truth is spoken in jest"
「冗談で真実を突く」という意味のフレーズ。
"I am saying it in jest, but it's not funny really".
(冗談だけど、笑えないよね)
It is believed that in every joke there is some underlying truth.
Therefore you can say this statement after you have the joke so that they know that although you were joking you were saying the truth(serious).
日本 
イギリス 
南アフリカ 
アメリカ合衆国 
