Add ID to translation

I have installed an Extension and FE is shown in the correct language.

Since I modified one template and want to add some text, I tried to add the related ID in the de.localllang.xlf - but it is not shown in FE.

<f:translate id="nextYear" />
      <trans-unit id="nextYear">
        <source>next year</source>
        <target state="needs-translation">nächstes Jahr</target>

Can someone help how I can add own IDs to the language file which then show up in its translation in the FE?
Regards Karl-Heinz

make sure you work with the correct translation file.
make sure you have cleared the cache (after the last edit to the translation file).

and you should insert the label in the default language, too.