YOU SAID:

Anyone who declares themselves victorious has probably done so because they've lost.

INTO JAPANESE

勝利を宣言する人は、おそらく負けたから勝利したのでしょう。

BACK INTO ENGLISH

Those who declare victory probably won because they lost.

INTO JAPANESE

勝利宣言した人は、負けたから勝ったのだろう。

BACK INTO ENGLISH

The person who declared victory must have won because he lost.

INTO JAPANESE

勝利宣言した人は負けたから勝ったに違いない。

BACK INTO ENGLISH

The person who declared victory must have won because he lost.