I noticed a few plugins having language files and supporting language strings for templates. So for example admin plugin and login plugin support languages. But many plugins don't have language files and don't support language strings for themes/templates.
Admin plugin has language strings in user/plugins/admin/languages.yaml. Login plugin has several language files in user/plugins/login/languages/ (e.g. en.yaml, de.yaml). As long as the templates support language strings the different setup of language files doesn't bother me (and as long as they are complete).
But now I try to adapt e.g. Antimatter theme for German strings and almost the complete template files have strings hardcoded in English. I created an inherited theme and already changed some template files to catch a translated string for output, like in <mytheme>/templates/partials/sidebar.html.twig
{{ "TAXONOMYLIST.POPULARTAGS"|t }}
I suppose creating and editing a language file in the plugin directory is subject to be overwritten when the plugin is updated. So I put my language strings in user/l anguages/en.yaml and user/languages/de.yaml but this way strings for all plugins are in one file per language. After all I feel a little irritated about how to handle language strings and files for plugins when I adapt a theme to support languages. I couldn't find an answer in the documentation so I thought asking would be appropriate :-)
Is there a naming convention for language strings? (like "<PLUGINNAME>.<STRING>")
Where should I put language files for plugins and how should I name them? (like user/languages/plugins/<PLUGINNAME>.yaml)