I came up with the following function, it seems to work well (need to
test a little more though).

Before I could submit a patch, I was wondering where I should place it:
it seems that placing it in guix/utils.scm triggers a whole world
rebuild.

Is there a way around it or should I send this patch to core-updates?

--8<---------------cut here---------------start------------->8---
(define* (make-desktop-entry-file destination #:key
                                  (type "Application") ; One of "Application", "Link" or "Directory".
                                  (version "1.1")
                                  name
                                  (generic-name name)
                                  (no-display #f)
                                  comment
                                  icon
                                  (hidden #f)
                                  only-show-in
                                  not-show-in
                                  (d-bus-activatable #f)
                                  try-exec
                                  exec
                                  path
                                  (terminal #f)
                                  actions
                                  mime-type
                                  (categories "Application")
                                  implements
                                  keywords
                                  (startup-notify #t)
                                  startup-w-m-class
                                  #:rest all-args)
  "Create a desktop entry file at DESTINATION.
You must specify NAME.

Values can be booleans, numbers, strings or list of strings.

Additionally, locales can be specified with an alist where the key is the
locale.  The #f key specifies the default.  Example:

  #:name '((#f \"I love Guix\") (\"fr\" \"J'aime Guix\"))

produces

  Name=I love Guix
  Name[fr]=J'aime Guix

For a complete description of the format, see the specifications at
https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html."
  (define (escape-semicolon s)
    (string-join (string-split s #\;) "\\;"))
  (define* (parse key value #:optional locale)
    (set! value (match value
                  (#t "true")
                  (#f "false")
                  ((?  number? n) n)
                  ((?  string? s) (escape-semicolon s))
                  ((?  list? value)
                   (catch 'wrong-type-arg
                     (lambda () (string-join (map escape-semicolon value) ";"))
                     (lambda args (error "List arguments can only contain strings: ~a" args))))
                  (_ (error "Value must be a boolean, number, string or list of strings"))))
    (format #t "~a=~a~%"
            (if locale
                (format #f "~a[~a]" key locale)
                key)
            value))

  (define key-error-message "This procedure only takes key arguments beside DESTINATION")

  (unless name
    (error "Missing NAME key argument"))
  (unless (member #:type all-args)
    (set! all-args (append (list #:type type) all-args)))
  (mkdir-p (dirname destination))

  (with-output-to-file destination
    (lambda ()
      (format #t "[Desktop Entry]~%")
      (let loop ((args all-args))
        (match args
          (() #t)
          ((_) (error key-error-message))
          ((key value . ...)
           (unless (keyword? key)
             (error key-error-message))
           (set! key
                 (string-join (map string-titlecase
                                   (string-split (symbol->string
                                                  (keyword->symbol key))
                                                 #\-))
                              ""))
           (match value
             (((_ . _) . _)
              (for-each (lambda (locale-subvalue)
                          (parse key
                                 (if (and (list? (cdr locale-subvalue))
                                          (= 1 (length (cdr locale-subvalue))))
                                     ;; Support both proper and improper lists for convenience.
                                     (cadr locale-subvalue)
                                     (cdr locale-subvalue))
                                 (car locale-subvalue)))
                        value))
             (_
              (parse key value)))
           (loop (cddr args))))))))
--8<---------------cut here---------------end--------------->8---

-- 
Pierre Neidhardt
https://ambrevar.xyz/