Fix broken language matching logic in TSMS

Checking language similarity with String#startsWith() is fundamentally
broken.  For instance, "fi" (Finnish) can match "fil" (Filipino).

This CL addresses such a broken comparison.

Bug: 110275412
Test: Manually verified that spell checker still works
Change-Id: I8d71760c534627b7707e16eb0fc648989f7692ae