Enforce String literal inside Localization.lang() calls

[stefan-kolb]

Introduced by the global search bar of Stupro.
We need to enforce that we use Strings or concatenated Strings inside our Localization.lang calls and no variables. otherwise our detection LocalizationConsistencyTest cannot work correctly.
Fixed by 05e7f0d.

We should accept a String in "" or a concatenated String " " +var + "" ... as it is required sometimes for insertions %0.

[stefan-kolb]

@JabRef/stupro @koppor Would be nice if Stupro can take care of this.