Your sentence has the main issue that it does not express what you want it to express. If you chose the correct wording, your question would be moot because it can no longer apply. A side-issue is the fact that keinen does not modify or belong to the verb in any way but is a modifier for the noun — in this case, Deutsche. It has to agree with the noun it modifies in numerus and genus but not with the verb and not necessarily with the subject. Let’s break it down:
‘I do not understand German’
Your choice of subject and verb is correct, however German, the language is das Deutsche or simply Deutsch. Since it is unmodified, so is kein:
Ich verstehe kein Deutsch.
You could also choose to word it differently, placing more emphasis on the not by saying:
Ich verstehe Deutsch nicht.
Deutsch, the language, is uncountable so kein before it can never be turned into the plural in any way. Also, naturally, the numerus of the object is independent of the numerus of the subject, so changing ich to wir only modifies the verb:
Wir verstehen kein Deutsch.
‘Ich verstehe keine Deutsche’
As stated above, this sentence is correct but it does not mean what you want it to mean. In standard German its meaning is ‘I don’t understand any German (person)’ with the noun being feminine, i.e. female German. Its meaning is not limited to understanding the language but could also mean ‘I can’t follow her way of thinking.’ It can be understood as you being sexist (‘Ich verstehe Deutsche aber keine Deutsche’) so if you are going to use it, do so with extreme caution. Making the subject plural again modifies the verb but does not modify the accusative object:
Wir verstehen keine Deutsche.
*‘Wir verstehen keinen Deutsche’
This sentence, as marked by the asterisk, is not correct. However, you can turn it into a correct one in two different ways:
This last version has transformed the accusative feminine singular keine Deutsche into an accusative masculine singular keinen Deutschen. So the sentence’s meaning is essentially ‘I don’t understand a single German.’ (Gendering enthusiasts will only understand it as ‘I don’t understand a single male German’, but from a generic and etymological point of view, the masculine genus is a standard genus.) Note again that verstehen can also mean following their way of thinking etc. Turn this sentence from wir to ich gives us the following:
Ich verstehe keinen Deutschen.
What about not understanding Germans?
Both these last examples are only correct if you are not speaking of Germans as a group. If you did, you would use the plural form in both English and German. The plural of ein Deutscher is especially complicated since it switches between strong and weak inflection: With kein or die, the noun is inflected weakly to become keine Deutschen/die Deutschen whereas lacking an article, it is inflected strongly to become Deutsche. Furthermore, in colloquial speech people may speak carelessly and incorrectly use the strong form even though there is a kein there. Long story short, the correct way to say ‘I don’t understand (any) Germans’ is:
Ich verstehe keine Deutschen.
Which may colloquially be turned into your first example sentence. Again, making the subject plural doesn’t modify the object, so with wir it is:
Wir verstehen keine Deutschen.
Again with the caveat that verstehen does not only mean to understand the language.
Strong inflection would be:
Ich verstehe Deutsche nicht.
Which is possible but has a different nuance.