The most popular language to speak in Germany is German. German is a language that has been around ever since Germany, basically. Other languages are spoken too - like English. There are variations of German. There is Old German, or High German, which is spoken in remote areas or by the older generations at times.
A whopping 95% of the German population speaks German in one form or another. Some speak Sorbian and North Frisian is also spoken. But for sure, German is the most popular first language in Germany. Many speak a second language, however.