Is the word "lead" in this sentence plural or singular

In your sentence, "lead" refers specifically to that of the Spanish, which is one instance thereof.

If you wanted to refer to all of the English and Spanish leads together you might have written, "currently we have 57 English and 1 Spanish leads in the task tracker."

No. 57 leads are a plural quantity; 1 is singular. you would never write "1 leads". However, fluent native speakers are likely to avoid the "leads... lead" repetition by omitting the second "lead":

Currently we have 57 English leads and 1 Spanish in the task tracker. ("Lead" is implied after "Spanish".)

Currently I have three red cars and one blue. ("Car" is implied after "blue".)

... or by doing this:

Currently we have 58 leads in the task tracker: 57 English and 1 Spanish.

Currently I have four cars: three red and one blue.

It is better to write numbers, amounts, etc in words in prose - three cars, ten tons, six years.

It would be "57 English and 1 Spanish lead".

You can have both nouns (plural and singular):

Fifty-seven English leads and one Spanish lead.

You can omit the plural noun and imply the singular one:

Fifty-seven English leads and one Spanish.

Or vice-versa:

One Spanish and fifty-seven English leads.