Add Turkmen localizations

[phillip-hopper]

[peteroatley]

@phillip-hopper - Thanks for doing this. I looked at https://www.webonary.work/turkmen/ and the new translations aren't showing.

The only thing I could see is that the language code from FLEx is 'tk-TM' and not 'tk'. I don't know if this makes a difference. The language code under Posts is also tk-TM.

[phillip-hopper]

@peteroatley, WordPress always uses the extended code, so it needs to be tk_TM. Even English is en_US, en_GB, en_CA.

[peteroatley]

The site seems to be setup correctly when I compare it to Naxi https://webonary.work/naxi which has Chinese and English.
But the translated strings aren't appearing when the Turkmen language is selected.