Simplify/generalise speller/manifest.ts

[snomos]

The function deriveLangTag is using a hard-coded list of languages to be converted from 3-letter to 2-letter codes. This information is already present in configure.ac for all languages, and should be taken from that file.

Since manifest.toml is derived by autoconf from manifest.toml.in, we can just use the variable @GLANG2@. We need to update manifest.toml.in to contain the necessary information, and then update the function to use the new information instead of the hard-coded list.

@GLANG2@ will be 2-letter if available, 3-letter otherwise. It is subject to human errors, though - the information is provided by humans upon repo creation time. The actual risk for error is pretty low, and is easy to correct if it happens.