Just consider this, there are more than 200 functioning Wikipedias and this is only possible because people localise the MediaWiki software in over 280 languages. It makes translatewiki.net, the website where all this work happens a strategic resource to the Wikimedia movement.
Internationalisation (i18n) and localisation (l10n) are an integral part of software development. It is an integral part of a continuous process and it requires constant attention. The day to day jobs are well in hand. The localisation itself is a community effort and with developers continually expanding the software base a continuous effort is needed of the translators to keep up with their language. This is hard and for many languages it is a struggle to keep up with even the "most used messages".
Managing this effort is a continuous effort, it is essential to maintain the i10n and the localisation optimally. It follows that it should be obvious what messages have the biggest impact first on the readers and then the editors of a Wikipedia. What should be in the "most used messages" changes over time and when it is considered strategic, such maintenance is to be considered a Wikimedia/MediaWiki undertaking.
Translatewiki has always been an independent partner of the Wikimedia Foundation and it has always been firmly part of the Wikimedia movement. Given that partnerships are a key part of the strategic plans of the WMF, the proof of the partnership pudding is very much in how it interacts with a translatewiki.net. TWN does not need to be part of the WMF organisation for it to fund TWN, it is clearly a quid pro quo. The WMF should even encourage TWN and other partners to collaborate for their i18n and l10n and enable this for strategic purposes, strengthening these partners globally.