What is the “Language” field in Section 1, and how is it different from the interface language switcher?

The Language field in Section 1 (Product Details) is a separate, required field that sets the language the SDS document's own content is written in: hazard statements, section headings, every phrase on the actual sheet.

It has 33 options (English, Arabic, Bulgarian, Chinese, Czech, Danish, Dutch, Estonian, Finnish, French, German, Greek, Hebrew, Hungarian, Indonesian, Italian, Japanese, Korean, Latvian, Lithuanian, Norwegian, Polish, Portuguese, Romanian, Russian, Slovak, Slovenian, Spanish, Swedish, Thai, Turkish, Ukrainian, and Vietnamese) and is completely independent of both the Country field next to it and the website's interface language switcher next to your avatar — you could set Country to United States and Language to French on the same SDS.

Short version: use the avatar switcher for your own browsing comfort; use this in-form field for what actually gets printed on the SDS.