Well, I suppose there are two ways to be 'good at processing text'. The first way is to quickly and efficiently let the user do what they want, which is how Python approaches the problem, eg, str[0:len(str)-1] + "'\''". I've only learned python in the last 12 months, and I need access to a lot less documentation than I need in elisp to write that.
The second way is to have a model that is so strong that it is worth the user learning that model (this is a rarer approach, but is more common in the lisp families - Clojure, for example, does this with variables). If elisp has this, it would be a good idea for someone to mention that in the documentation, because I don't recall ever seeing anyone say "Wow! This elisp model of processing text is amazing! I've been doing it wrong my whole life!". I have seen variants of "which is not at all bad, but is very foreign".
I don't know of a best language, but manipulating text strings is the foundation of most web serving. So I'd say that the popular languages for that (python, perl, php, etc) are quite a bit better than elisp at processing text.
This actually takes it in a direction I disagree with.
Once you get used to manipulating buffers, elisp actually is easier than most other languages. What it is bad at is processing strings. Because it is good at processing text buffers.
I am on my phone, but if this thread is still going this evening, I'll show roughly what that snippet would look like. Will be, roughly, (forward-word) (move -1) (insert "/"). Obviously, some differences if you were moving words or paragraphs. But basic idea is the same.
That is, don't try and manipulate strings. Manipulate the entire buffer. If needed, build a new buffer with what all you want.
The irony is that elisp has a decent dsl for manipulating text. But you have to embrace the mutable nature of it. Which is unexpected for many working with a lisp.
TXR Lisp is a dialect that has decent features for munging text. It's part of a two-language combination called TXR; the other language is the TXR Pattern Language: a notation for matching small and large scale structures in text documents and streams.
TXR Lisp allows various classic Lisp operations to also be used on vectors and strings (think car, cdr, mapcar* and numerous others). It has array referencing that support Python's negative index convention. Sequence subranges are assignable places, so if s holds "firetruck" then (set [s 1..-1] "lic") changes s to "flick".
TXR Lisp is loaded with useful features, all packed into a small executable accompanied by a small library of Lisp files.
The second way is to have a model that is so strong that it is worth the user learning that model (this is a rarer approach, but is more common in the lisp families - Clojure, for example, does this with variables). If elisp has this, it would be a good idea for someone to mention that in the documentation, because I don't recall ever seeing anyone say "Wow! This elisp model of processing text is amazing! I've been doing it wrong my whole life!". I have seen variants of "which is not at all bad, but is very foreign".
I don't know of a best language, but manipulating text strings is the foundation of most web serving. So I'd say that the popular languages for that (python, perl, php, etc) are quite a bit better than elisp at processing text.