make should throw warnings about language names with no native variant

When html-tools is compiled (make), it gets localised language names from the APy instance. It's supposed to fall back on the native name for a language when a local one doesn't exist.

Since it's nearly impossible to maintain each language name in each localised language, we should *always* have the native names available. To that end, html-tools should throw a warning for any language that doesn't have at least its native name available.

See also https://github.com/apertium/apertium-apy/issues/152 and https://github.com/apertium/apertium-apy/issues/153.