The best way to promote the use of the Welsh language in Wales (where a huge preponderance of people living here cannot speak the language) is to avoid alienating the majority language users.
Whilst Welsh is the historic language of Wales, the current position is radically different. If, for example, job seeking English language speakers start to clock that, where public sector jobs are concerned, there is a caste system developing, there will be a reaction.
Accordingly, I would concentrate on promoting Welsh in the Arts, developing Welsh language schools where there is demand, try to get a healthy Welsh press and Welsh social media scenario going, and so on. I would avoid, however, using the approach that many are now talking about of getting “muscular” with the language