YOU SAID:
The rain in Spain stays mainly in the plain.
INTO JAPANESE
スペインの雨は主に平地滞在します。
BACK INTO ENGLISH
The rain in Spain stays mainly flat.
INTO JAPANESE
スペインの雨は主に平らにとどまります。
BACK INTO ENGLISH
The rain in Spain stays mainly flat.
That didn't even make that much sense in English.