Google Alerts does not support tracking a keyword in multiple specific languages with one alert. However, you can work around this limitation by setting up two separate Google Alerts, one for each language.
To provide more context, Google Search does not have a way to specify the language as part of the search query. You can verify this by using Google's Advanced Search. It has a similar interface to Google Alerts, where you can select a single language. Then, when you search, you can look at the query string that will look like the following:
https://www.google.com/search?as_q=test&lr=lang_de
Notice that test
is what was searched for, and lang_de
specifies that the search is restricted to German results. The point is that the search term and language are specified as two separate parameters in the query string and that there is no way to modify the query string so that it will return a specific language.