[netinstall] Improve documentation of translation framework

- Not country-code, but language-code

FIXES #1411

src/modules/netinstall/netinstall.conf

@@ -83,11 +83,12 @@ required: false
 # and existing translations. If no *title* values are provided, no string
 # is displayed.
 #
-# Translations are handled through [cc] notation, much like in
-# `.desktop` files. The string `key[cc]` is used for *key* when
-# when the language *cc* (country-code, like *nl* or *en_GB*) is used.
+# Translations are handled through `[ll]` notation, much like in
+# `.desktop` files. The string associated with `key[ll]` is used for
+# *key* when when the language *ll* (language-code, like *nl* or *en_GB*
+# or *ja*) is used.
 #
-# The following strings are already known to Calamares and can be
+# The following strings are **already** known to Calamares and can be
 # listed here in *untranslated* form (e.g. as value of *sidebar*)
 # without bothering with the translations: they are picked up from
 # the regular translation framework:
@@ -110,9 +111,12 @@ required: false
 #  - "Theming"
 #  - "Gaming"
 #  - "Utilities"
+# Other strings should follow the translations format.
 label:
     sidebar: "Package selection"
     # sidebar[nl]: "Pakketkeuze"
+    # sidebar[en_GB]: "Package choice"
+    # sidebar[ja]: "知りません"  # "I don't know"
     # title: "Office Package"
     # title[nl]: "Kantoorsoftware"