Reuse S and SLEN macros in drawstr macro

A minor step toward decoupling from particular i18n implementation.
Split i18n into separate files per language

This separates different languages into self-contained files,
with strict order-independent ID->STRING correspondence,
automatic fallback to English for untranslated IDs, and
a syntax closely resembling (but not really valid)
gettext's PO files, in case we ever migrate to it.

Also, two new IDs are added for future use:
LANG_NATIVE_NAME for the language name in the language itself,
and LANG_ENGLISH_NAME for the language name in English.
These are expected to be used e.g. in language choice menu.
Eventually.
Stop relying on hardcoded size for i18n tables
Merge branch 'better-i18n' of https://github.com/albel727/uTox

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Fixed warnings, added DEFAULT_LANG define set to english.

This seems like a better way to set untranslated lines to english.