I have the following button that I want to translate: (highlighted with yellow)
The translations are maintained in transaction SOTR_EDIT (for DE, EN and ES), but if I logon in EN/ES, I still see the text in DE.
The only difference between this button and the rest (which are correctly translated) is the package to which they are assigned.
Is this relevant? Where should I check more? In ST05 / ST12 traces, I did not find any relevant data.

First point. Check if you are deal with the text you are intended to. There can be multiple texts with the same name in different packages. Check all.
Second. Go to translation workbench (
SE63tcode) and search for you text by fetching all the text through package name + wildcardFind your text by concept which you can take from
SOTR_EDITand check if it really translatedIf there is red color like here then translation is missing