The construction below looks counter-intuitive, but it works:
Given:
Tags = ['item1', 'item2', 'item3']
This:
{{ range $i, $e := .Params.tags }}{{ if $i }}, {{ end }}{{ $e }}{{ end }}
Gives:
item1, item2, item3
The example above uses tags with comma as delimiter, but this of course also applies to other ranges and delimiters.
This worked out great. The only thing I can’t figure out now is how to handle the trailing comma in a key:value map.
This is a partial (array_to_sentence_string) that I translated from some code in Jekyll.
{{ $len := len . }}{{ if eq $len 1 }}{{ index . 0 }}{{ else if eq $len 2 }}{{ index . 0 }} and {{ index . 1 }}{{ else }}{{ $last := sub $len 1 }}{{ range first $last . }}{{ . }}, {{ end }}and {{ index . $last }}.{{ end }}
You use it as {{% array_to_sentence_string .Params.tags %}}. This results in:
item1, item2, and item3.
I also have partials/post/tag/list.html (in cabaret) that more closely does what you want.
{{ with .Params.tags }}
<div class="tags-list">
<span class="dark-red">Tags</span><span class="decorative-marker">//</span>
{{ $len := len . }}
{{ if eq $len 1 }}
{{ partial "post/tag/link" (index . 0) }}
{{ else }}
{{ $last := sub $len 1 }}
{{ range first $last . }}
{{ partial "post/tag/link" . }},
{{ end }}
{{ partial "post/tag/link" (index . $last) }}
{{ end }}
</div>
{{ end }}
I just submitted a PR with a new delimit function that handles most of these use cases in a much more readable fashion (not the and separator yet).
I wanted to use the new delimiter function, but I don’t know how to add link around tags. Is it possible ?
In the meantime, the solution provided by @bep at the beginning of this thread works fine. So thanks for that. 
It is possible, but it is a little…unconventional. You have to apply around. This is what I use in cabaret for my tag list.
<!-- layouts/partials/post/tag/list.html -->
{{ with .Params.tags }}
<div class="tags-list">
<span class="dark-red">Tags</span><span class="decorative-marker">//</span>
{{ $sort := sort . }}
{{ $links := apply $sort "partial" "post/tag/link" "." }}
{{ $clean := apply $links "chomp" "." }}
{{ delimit $clean ", " }}
</div>
{{ end }}
This block:
Sorts .Params.tags and assigns it to $sort.
sort .Params.tags => sort [ c, a, b ] => [ a, b, c ]
Applies the function partial over the elements of $sort and assigns the resulting list to $links. The partial function is given post/tag/link as the name of the partial to use, and "." is the placeholder for the current element during function application.
apply [ a, b, c ] "partial" "post/tag/link" "." => [
partial "post/tag/link" "a",
partial "post/tag/link" "b",
partial "post/tag/link" "c",
] => [
<a class="post-tag post-tag-a" href="/tags/a">a</a>\n,
<a class="post-tag post-tag-b" href="/tags/b">b</a>\n,
<a class="post-tag post-tag-c" href="/tags/c">c</a>\n,
]
Removes the trailing \n from each resulting link in $links by applying chomp over the elements and assigning it to $clean.
apply [ <a…>a</a>\n, <a…>b</a>\n, <a…>c</a>\n ] "chomp" "." => [
<a class="post-tag post-tag-a" href="/tags/a">a</a>,
<a class="post-tag post-tag-b" href="/tags/b">b</a>,
<a class="post-tag post-tag-c" href="/tags/c">c</a>,
]
Creates a single result by delimiting $clean with ", "; this is inserted as the tag list.
delimit [ <a…>a</a>, <a…>b</a>, <a…>c</a> ] ", " =>
"<a…>a</a>, <a…>b</a>, <a…>c</a>"
Like I said, it’s a little unconventional, and it requires a little bit of preparatory work to make it sensible, but it’s quite powerful.
<!-- layouts/partials/post/tag/link.html -->
<a class="post-tag post-tag-{{ . | urlize }}" href="/tags/{{ . | urlize }}">{{ . }}</a>
Well, indeed, it’s not conventional… 
Don’t you think there should be an easy way ? A function that would output directly a list of taxonomy and links. Maybe I should open a feature request for that.
Thanks anyway for taking the time to explain your method !