When I create a new content type in WEB-INF/param/content-types/[my_plugin]/[content-type-id].xml, the file looks this :
<?xml version="1.0" encoding="UTF-8" ?>
<cms:content-type xmlns:cms="http://www.ametys.org/schema/cms">
<cms:label i18n="true">CONTENT_CUSTOM_LABEL</cms:label>
<cms:description i18n="true">CONTENT_CUSTOM_DESCRIPTION</cms:description>

<cms:metadata name="title" type="string">
<label i18n="true">CONTENT_CUSTOM_TITLE</label>
<description i18n="true">CONTENT_CUSTOM_TITLE_DESC</description>
<validation>
<mandatory />
</validation>
</cms:metadata>

<!-- [...] -->
</cms:content-type>

In the ribbon, we could see the CONTENT_CUSTOM_LABEL and CONTENT_CUSTOM_DESCRIPTION correctly translated but in the edit mode of the content type, the CONTENT_CUSTOM_TITLE and CONTENT_CUSTOM_TITLE_DESC are not translated. There are preceding by the mention : plugin.[my_plugin]: like plugin.[my_plugin]:CONTENT_CUSTOM_TITLE.

In the documentation (https://wiki.ametys.org/display/AMETYSV3/Creating+a+new+content+type), the default dictionary should be located in WEB-INF/i18n, that's what I do.

In consequence, there is a inconsistency between the default dictionnary for the ribbon menu and the default dictionnary for the edit mode of the new content type.