Out of the provided options, the most appropriate word to complete the sentence is "pick up":
Did it take you long to pick up Spanish?
This phrase accurately conveys the idea of acquiring basic knowledge or proficiency in the language. The other options wouldn't fit as well in this context:
Look up: This implies searching for specific information about Spanish, not necessarily learning the language itself.
Take: While "take" can be used in some contexts related to language learning (e.g., "take a Spanish class"), it's not the most natural choice here.
Send: Sending something implies transmitting information, not acquiring it, so it wouldn't be suitable in this case.