What is the maximum SMS message length when sent through the Clickatell API for English and Spanish messages?
Is there is a difference between English and Spanish message lengths, since Spanish may contain Unicode characters?
From the SMS wikipedia page:
Messages are sent with the MAP MO- and MT-ForwardSM operations, whose payload length is limited by the constraints of the signaling protocol to precisely 140 octets (140 octets = 140 * 8 bits = 1120 bits).
Depending on which alphabet the subscriber has configured in the handset, this leads to the maximum individual short message sizes of 160 7-bit characters, 140 8-bit characters, or 70 16-bit characters.
As for your other question:
Is there is a difference between English and Spanish message lengths, since Spanish may contain Unicode characters?
No, there is no difference, as both English and Spanish are completely covered in the 8-bit Latin 1 character set.