I recall a conversation I had with a coworker years ago. This fellow mentioned he thought differences between languages (e.g., Perl and PHP) were simple syntax.
This is true, sort of. The nuts and bolts for each language (displaying a variable’s value and the like) are same in concept and similar in construct.
<?php =someVar ?>
<% puts someVar %>
You get the idea.
The difference (and where the coworker was wrong), is the concept of how the data flows through the system and which pattern(s) are encouraged/discouraged.
Some languages encourage MVC, whereas others encourage spaghetti and still others encourage a hybrid (or something else entirely, like modules).
What is a language best at?
Going beyond this, what is the language best at doing? Is it installed in places you want to host your code? Do you have data structures/stores that fit better with one language than another?
Picking the best tool for the job is important in delivering a quality product. Choosing which language to use isn’t trivial and can make it easier—or harder—to deliver the solution.