.htaccess redirect domain to subdomain

Each of the two languages shall have it's own subdomain. In my case German (de.domain.com) and English (en.domain.com).
Default language is supposed to be English. Therefore, I plan to redirect all requests (www.domain.com and domain.com) to the en.domain.com subdomain.

Externally redirect ^(www\.)?domain.com/<anything> to /en.domain.com/<anything>

The different language contents will be placed in language folder domain.com/en/ and domain/de/ in domain root.

Internally rewrite <two-letters>.domain.com/ to /<two-letters>

I also want to Rewrite Extensionless URLs for HTML and PHP files.

Internally rewrite requests for /<anything-with-no-period-in-last-part-of-path> to </<anything-with-no-period-in-last-part-of-path>.html if </<anything-with-no-period-in-last-part-of-path>.html exists (see RewriteCond "-f" flag) AND
rewrite requests for /<anything-with-no-period-in-last-part-of-path> to </<anything-with-no-period-in-last-part-of-path>.php if </<anything-with-no-period-in-last-part-of-path>.php exists

See the "Cruft-free URLs" thread in our Apache Forum Library.

I also want to force a trailing slash on all folders

Done automatically by Apache mod_dir

I can't see the wood for the trees anymore.

Break the problem into smaller pieces, and work on only one piece at a time. (Take a few steps back from the forest so that you can see only a few trees).

Jim