Lookup in-tree templates for each language in search (Fixes #535)
- Until now, the in-tree templates were only taken into account after
custom templates for all preferred languages were scanned. This is
changed now: at the end of each language, a search for in-tree
templates for that language is added. This fixes two separate issues:
- If the list of preferred languages in a search contained first english (but no custom english templates existed) and then another language with existing custom templates, then the custom template of that second language was picked. But the in-tree english template should have had priority.
- If mailman later ships in-tree templates for other languages, they will be taken into account.