Is using MySQL views a good translation mechanism? We want to use a foreign vendor MySQL database, but the whole schema is barely understandable by our development team. We could alter the schema to rename tables and columns once for all, but the database is periodically updated and reinstalled on our production servers.

Would building isomorphic views be a good translation mechanism in our case? We would use our locale in application queries and yet be able to refer to native locale in case we wish to interact with the database vendor.

As we would have strictly the same tables and columns, would there be any performance alteration, in particular with regards to existing indexes?
Views are a good way to accomplish what you want to do. If your views take advantage of existing indexes or are 1:1 against the underlying tables, then queries against them will use the indexes. Since you're expecting periodic updates, you'll want to script the view creation, and in that same deployment script you can always add indexes if you need more.

You will want a 1:1 view of the columns for each table that simply performs the translation on the column and table names, enabling updates and inserts on those views in your locale. If the database contains stored procedures, you can create wrappers for them in your native language by adding new procedures to accept translated parameters and call the original procedures.

One caveat is the collation used by the database: if it is not suitable for your locale, using COLLATE to a different character set in your SQL statements will negatively impact performance.