I researched the language distribution using the Web of Science. Since your interest was in new content, I restricted the search to the period 1985-2015. The total number of mathematics articles in that database is 882565 for this period. Here is the breakdown by language:
http://ilorentz.org/beenakker/MO/language_1985.png
[1985-2015]
The Web of Science goes back to 1945, so I also made the search for the entire period 1945-2015. That gives a total of 1120170 mathematics articles, with this breakdown by language:
http://ilorentz.org/beenakker/MO/language_1945.png
[1945-2015]
Finally, for the last ten-year period 2005-2015 the breakdown (for a total of 459213 mathematics articles) is
http://ilorentz.org/beenakker/MO/language_2005.png