Hi,
I've worked on Bug #1483930 and resolved it thus:
There are now two additional labels named "previousmessage" and 
"nextmessage", which are used in message.html, as opposed to 
"previousmessages" and "nextmessages" in mail.html.
I've included the label entries for en_GB, en_US, de_DE, de_CH and es in 
the patch.
If that's an okay course of action for you, please commit the patch and 
close the ticket :)
Cheers,
mtu
-- 
What seems to be the Officer, problem?
- From South Park, "Bloody Mary"
diff -uNr rc-294/program/localization/de_CH/labels.inc rc-294fix/program/localization/de_CH/labels.inc
--- rc-294/program/localization/de_CH/labels.inc	2006-08-06 15:51:22.000000000 +0200
+++ rc-294fix/program/localization/de_CH/labels.inc	2006-08-06 17:15:07.000000000 +0200
@@ -99,7 +99,9 @@
 $labels['deletemessage']    = 'In den Papierkorb verschieben';
 $labels['printmessage']     = 'Diese Nachricht drucken';
 $labels['previousmessages'] = 'Vorherige Nachrichten anzeigen';
-$labels['nextmessages']     = 'Weitere Nachrichten anzeigen';
+$labels['previousmessage']  = 'Vorherige Nachricht anzeigen';
+$labels['nextmessages']     = 'Folgende Nachrichten anzeigen';
+$labels['nextmessage']      = 'Folgende Nachricht anzeigen';
 $labels['backtolist']       = 'Zurück zur Liste';
 $labels['select'] = 'Auswählen';
diff -uNr rc-294/program/localization/de_DE/labels.inc rc-294fix/program/localization/de_DE/labels.inc
--- rc-294/program/localization/de_DE/labels.inc	2006-08-06 15:51:20.000000000 +0200
+++ rc-294fix/program/localization/de_DE/labels.inc	2006-08-06 17:03:49.000000000 +0200
@@ -100,7 +100,9 @@
 $labels['deletemessage']    = 'In den Papierkorb verschieben';
 $labels['printmessage']     = 'Diese Nachricht drucken';
 $labels['previousmessages'] = 'Vorherige Nachrichten anzeigen';
-$labels['nextmessages']     = 'Weitere Nachrichten anzeigen';
+$labels['previousmessage'] =  'Vorherige Nachricht anzeigen';
+$labels['nextmessages']     = 'Folgende Nachrichten anzeigen';
+$labels['nextmessage']     =  'Folgende Nachricht anzeigen';
 $labels['backtolist']       = 'Zurück zur Liste';
 $labels['select'] = 'Auswählen';
diff -uNr rc-294/program/localization/en_GB/labels.inc rc-294fix/program/localization/en_GB/labels.inc
--- rc-294/program/localization/en_GB/labels.inc	2006-08-06 15:51:20.000000000 +0200
+++ rc-294fix/program/localization/en_GB/labels.inc	2006-08-06 16:56:16.000000000 +0200
@@ -99,7 +99,9 @@
 $labels['deletemessage']    = 'Move message to trash';
 $labels['printmessage']     = 'Print this message';
 $labels['previousmessages'] = 'Show previous set of messages';
+$labels['previousmessage']  = 'Show previous message';
 $labels['nextmessages']     = 'Show next set of messages';
+$labels['nextmessage']      = 'Show next message';
 $labels['backtolist']       = 'Back to message list';
 $labels['viewsource']       = 'Show source';
diff -uNr rc-294/program/localization/en_US/labels.inc rc-294fix/program/localization/en_US/labels.inc
--- rc-294/program/localization/en_US/labels.inc	2006-08-06 17:09:45.000000000 +0200
+++ rc-294fix/program/localization/en_US/labels.inc	2006-08-06 16:58:43.000000000 +0200
@@ -98,7 +98,9 @@
 $labels['deletemessage']    = 'Move message to trash';
 $labels['printmessage']     = 'Print this message';
 $labels['previousmessages'] = 'Show previous set of messages';
+$labels['previousmessage']  = 'Show previous message';
 $labels['nextmessages']     = 'Show next set of messages';
+$labels['nextmessage']      = 'Show next message';
 $labels['backtolist']       = 'Back to message list';
 $labels['viewsource']       = 'Show source';
diff -uNr rc-294/program/localization/es/labels.inc rc-294fix/program/localization/es/labels.inc
--- rc-294/program/localization/es/labels.inc	2006-08-06 15:51:22.000000000 +0200
+++ rc-294fix/program/localization/es/labels.inc	2006-08-06 17:05:28.000000000 +0200
@@ -103,7 +103,9 @@
 $labels['deletemessage']    = 'Mover mensaje a la papelera';
 $labels['printmessage']     = 'Imprimir este mensaje';
 $labels['previousmessages'] = 'Mostrar mensajes anteriores';
+$labels['previousmessage']  = 'Mostrar mensaje anterior';
 $labels['nextmessages']     = 'Mostrar mensajes siguientes';
+$labels['nextmessage']      = 'Mostrar mensaje siguiente';
 $labels['backtolist']       = 'Volver a la lista de mensajes';
 $labels['viewsource']       = 'Mostrar código';
diff -uNr rc-294/skins/default/templates/message.html rc-294fix/skins/default/templates/message.html
--- rc-294/skins/default/templates/message.html	2006-08-06 15:51:29.000000000 +0200
+++ rc-294fix/skins/default/templates/message.html	2006-08-06 17:26:15.000000000 +0200
@@ -11,9 +11,9 @@
 <roundcube:include file="/includes/header.html" />
 <div id="messagecountbar">
-<roundcube:button command="previousmessage" imageSel="/images/buttons/previous_sel.png" imageAct="/images/buttons/previous_act.png" imagePas="/images/buttons/previous_pas.png" width="11" height="11" title="previousmessages" />
+<roundcube:button command="previousmessage" imageSel="/images/buttons/previous_sel.png" imageAct="/images/buttons/previous_act.png" imagePas="/images/buttons/previous_pas.png" width="11" height="11" title="previousmessage" />
  <roundcube:object name="messageCountDisplay" /> 
-<roundcube:button command="nextmessage" imageSel="/images/buttons/next_sel.png" imageAct="/images/buttons/next_act.png" imagePas="/images/buttons/next_pas.png" width="11" height="11" title="nextmessages" />
+<roundcube:button command="nextmessage" imageSel="/images/buttons/next_sel.png" imageAct="/images/buttons/next_act.png" imagePas="/images/buttons/next_pas.png" width="11" height="11" title="nextmessage" />
 </div>
 <div id="messagetoolbar">