How to help parents get a reunion visa to Germany with a long-term residence visa in Germany (parents are too old to get language proof)?

You should pay attention to the requirements of the embassy:

In principle, "A 1 Language Certificate" ("A 1-Sprachzertifikat") must be used to prove the applicant's German language ability. The certificate must be issued by an examination institution with ALTE membership. At present, the Committee members include Goethe-Institut, gGmbH Telc, Austrian German Language Certificate Examination Center (? SD) and testing daf-institution.

With these three words in principle, there is room for discussion. The key to a visa is the local immigration office in your city. It's their confirmation letter to the embassy, allowing your parents to enter the country, which is what we often say. Can your parents communicate in English? It would be great if you could. There is also the most important guarantee, which must be long-term residence or German nationality. There is enough margin and credibility to ensure that your parents come. It's better for the spouse to be German, and let the spouse go to SAFE as a guarantee to send an invitation, and make that kind of invitation with financial guarantee.