Add language filtering for command line tools

[phlax]

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[phlax]

@dwaynebailey im not sure if this is necessary

Its already possible to do eg pootle fs tutorial sync_translations -P /en*

Also if we add a lang filter - do the -P and -p options still work?

[dwaynebailey]

@phlax if we had --language=zh-Hant it would allow any language mapping to make the correct decisions. But it might not be critica here as you say. I'd deprioritise this unless we have a stronger use case.