HCI (Human-Computer-Interaction) is never easy. Thinking of HCI and different languages, cultures, etc. while considering different user types and their knowledge + behaviour becomes very difficult. You don't want to confuse your users and every piece of information should be shown as understandable as possible.
There are a few things to consider when coding a web form. But the most important ones in this case are
- What pieces of information will be send?
- How long does it take?
Especially the second one is important, because while sending a large file via web form, the user should be informed that it takes a while and its still sending. As well as be informed, when the whole form has been send. If you don't the user might think, that the form does nothing, refreshes the site and unintentionally cancels the upload.
In the last few years some coding standards can be seen. Most notable:
a) Disable your send button and put an (animated e.g. three moving dots) "sending" (or anything like it) label in it.
b) Blur the whole screen and make it unaccessible for user input. Put some animated gif or text in the center of the screen while uploading.
Whatever you choose: After transmitting the form show a text informing the user, that the form has been succesfully send or an error message when it failed.
Using b) has an advantage. You don't have to use any translations (of course you can add text to clear things up). Using a animated rotating circle is wildely
aknowledged for an device which is "working" and "doing" something.
But lets get back to your question. For german web forms its quite ok to label the send-button
"Senden" and when pressed you may change it so "Sende" which is an abbreviation of "Ich sende" (I am transmitting).
As said above you may add animated dots ... after "Sende" to indicate, that the form is doing something right now.
Other possible translations are as @Pollitzer says
One last remark
German laws for online business transactions are quite complex. For example an online shop has to label its "Order"-button with something like "Zahlungspflichtig bestellen" (agreement to pay for the order you are placing). So while labeling a form button with "Senden" for transmitting a simple contact-form is the right way, but it may be the wrong word for more complex business cases.