1) The main issue:

At the end of https://documentation.magnolia-cms.com/display/DOCS/Types+of+translatable+text we recommend that app designers keep i18n messages for UI and for templates into two separate .properties files in a message bundle. This best practice is not followed in Travel Demo included in Magnolia 5.4.6 where both the UI messages and the Template messages are stored in one file:

module-travel-demo-messages_en.properties

2) A corollary to this issue is that
in i18n documentation we should eventually start recommending the designers to use a specific infix in the filename which will hold template massages, e.g.

<bundle-name><templates><locale>.properties

to distinguish them from UI messages, which could remain in

<bundle-name>_<locale>.properties

(Alternatively, to make it consistent, the recommended filename for UI messages could be
<bundle-name><UI><locale>.properties )

3) Furthermore
if the suggestion in 2) is adopted, the bundle of DE messages for Travel Demo should logically also include two files:

module-travel-demo-messages_de.properties
module-travel-demo-messages_templates_de.properties

4) A side note:
I miss

navigation.login=Log in
navigation.logout=Log out
navigation.registration=Register

in
lang/de/src/main/resources/mgnl-i18n/travel-demo/module-travel-demo-messages_de.properties

Shouldn't this file also include template keys like

navigation.login=Anmelden
navigation.logout=Abmelden
navigation.registration=Konto erstellen