a: expired certificate

Dialog text:
The signature is invalid: The signing certificate has expired.

Tooltip:
If the certificate was still valid when the file was signed, the signature may still be trustworthy. Check the signature date to decide.

b: revoked certificate

Dialog text:
The signature is invalid: The signing certificate was revoked.

Tooltip:
A revoked certificate could mean it was compromised. Only trust this signature if you're sure why the certificate was revoked.

c: untrusted certificate

Dialog text:
The signing certificate is not certified by a trusted person.

Tooltip:
This means Kleopatra can't confirm who signed the file. You can choose to trust the certificate or import a trusted certification.

After further discussion, I propose the following. All tool tips and the last dialog text were changed:

a: expired certificate
Dialog text:
The signature is invalid: The signing certificate has expired.

Tooltip:
Either obtain an updated certificate or check if the signature was made before the keys expiration and decide if you can trust it in this case.

b: revoked certificate
Dialog text:
The signature is invalid: The signing certificate was revoked.

Tooltip:
A revoked certificate could mean it was compromised. Only if the revocation reason was another one (e.g. replacement) then signatures made before the revocation may still be trustworthy.

c: untrusted certificate
Dialog text:
The signing certificate is not certified by yourself or a trusted entity.

Tooltip:
This means you can't be sure who signed the file. For that you need to establish trust in the certificate.

a: expired certificate
Dialog text:
The signature is invalid: The signing certificate has expired.

OK :)

Tooltip:
Either obtain an updated certificate or check if the signature was made before the keys expiration and decide if you can trust it in this case.

Korrektes Englisch wäre:
Either obtain an updated certificate or check whether the signature was made before the certificate’s expiration, and decide if you can trust it.

Das klingt allerdings recht holprig. Gegenvorschlag:
You can check if the signature was made before the certificate expired. If so, it may still be valid. Alternatively, ask the sender for an updated certificate.

b: revoked certificate
Dialog text:
The signature is invalid: The signing certificate was revoked.

OK :)

Tooltip:
A revoked certificate could mean it was compromised. Only if the revocation reason was another one (e.g. replacement) then signatures made before the revocation may still be trustworthy.

Vorschlag (klingt flüssiger): A revoked certificate may indicate it was compromised. If it was revoked for another reason (like replacement), signatures made before that may still be OK.
Oder deutlich kürzer: Revoked certificates are often unsafe. If it was replaced, earlier signatures may still be valid.

c: untrusted certificate
Dialog text:
The signing certificate is not certified by yourself or a trusted entity.

Besser: The signing certificate is not certified by you or a trusted person.

Tooltip:
This means you can't be sure who signed the file. For that you need to establish trust in the certificate.

Besser: The signer’s identity can’t be verified. You can trust the certificate manually or import a trusted certification.
Oder kürzer: Without trust, the signer’s identity can’t be confirmed.

Besser: The signing certificate is not certified by you or a trusted person.

A trusted key or a Root CA is no trusted person.

next version:

a: expired certificate
Dialog text:
The signature may be invalid: The signing certificate has expired.
Tooltip:
Ask the sender for an updated certificate. If you can assure that the signature was made before the expiration date, you may trust it.

b: revoked certificate
Dialog text:
The signature may be invalid: The signing certificate was revoked.
Tooltip:
A revoked certificate may indicate it was compromised. If it was revoked for another reason (like replacement), signatures made before the revocation date may still be OK.

c: untrusted certificate
Dialog text:
The signing certificate is not certified by you or a trusted third party.
Tooltip:
Without established trust, the signer’s identity can’t be confirmed.

The last needs a bit more info imho…

tooltip suggestion for d, not trusted and expired:
Ask the sender for an updated certificate and when you receive it, follow the procedure to establish trust and certify it.
or:
Ask the sender for an updated certificate. When you receive it, you need to establish trust and certify it.