Latin

Tenet insanabile multos scribendi cacoethes¹ Juvenal, Satires

The most common script in Britain, and indeed much of the wider world - used not just for the language of the Romans, but many others in its orbit.

This dispersion of uses does lead to some differences between regional versions², but it’s the most likely script that two people picked arbitrarily would be able to read and write to each other in. If you learn one script, it’s probably this one.

Footnotes

1. The incurable itch to write affects many. ↩

2. And just knowing the letters can leave you with no understanding of the meaning and only a close approximation of the pronunciation if the language of the text is not one you know. ↩