SharePoint language packs are per site basis. Two users cannot use different languages on the same site, and it is up to the site creator (upon site creation) to decide which language will be used. In your case you can benefit from language packs for local sites, that will be used only in one language, all the global sites should remain with English templates.
To be honest, if users do understand English you should stick to it. Non-English templates can be a problem for you cause all the error messages have been translated. Finding the solution for some weird error in small European language (e.g. Croatian) might be a challenge.
Number of people reported to me that quality of translations is poor. I personally have more problem understanding my native language pack (Croatian) than English one.
So my best practice: Customer can have SharePoint in any language so long as it is English :))))