Clarify language lists semantics

[tzdybal]

Language lists can be a counter intuitive/confusing, because of the "fallback" logic employed when given language(s) are absent. For example there is no way to select a value only in selected language(s), without fallback.

Proposed change is to add . as a special value in language list, denoting look for default value (no language tag) and then for value in any language.

New rules will be following:

• name => Look for default, then return nothing.
• name@. => Look for default, then any language.
• name@en => Look for en, then return nothing.
• name@en:. => Look for en, then default, then any language.
• name@en:pl => Look for en, then pl, then return nothing.
• name@en:pl:. => Look for en, then pl, then default, then any language.

[MichaelJCompton]

@tzdybal this makes lots of sense. Fixes the problems I was having in #994.

I'm wondering though if the semantics for name@en should just be to check in en - because there is no way in the proposed semantics to check if there is a string tagged with en.

[tzdybal]

@MichaelJCompton thanks for suggestion. Comment updated.